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device: Basic config and setup to support async I/O.
This commit is contained in:
Alasdair G Kergon
parent
7a9af3cd0e
commit
8c7bbcfb0f
@ -1,5 +1,6 @@
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Version 2.02.178 -
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=====================================
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Detect asynchronous I/O capability in configure or accept --disable-aio.
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Add AIO_SUPPORTED_CODE_PATH to indicate whether AIO may be used.
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Configure ensures /usr/bin dir is checked for dmpd tools.
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Restore pvmove support for wide-clustered active volumes (2.02.177).
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@ -59,6 +59,11 @@ devices {
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# This configuration option is advanced.
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scan = [ "/dev" ]
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# Configuration option devices/use_aio.
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# Use linux asynchronous I/O for parallel device access where possible.
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# This configuration option has an automatic default value.
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# use_aio = 1
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# Configuration option devices/obtain_device_list_from_udev.
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# Obtain the list of available devices from udev.
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# This avoids opening or using any inapplicable non-block devices or
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69
configure
vendored
69
configure
vendored
@ -707,7 +707,9 @@ FSADM
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ELDFLAGS
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DM_LIB_PATCHLEVEL
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DMEVENTD_PATH
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AIO_LIBS
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DL_LIBS
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AIO
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DEVMAPPER
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DEFAULT_USE_LVMLOCKD
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DEFAULT_USE_LVMPOLLD
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@ -954,6 +956,7 @@ enable_profiling
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enable_testing
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enable_valgrind_pool
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enable_devmapper
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enable_aio
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enable_lvmetad
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enable_lvmpolld
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enable_lvmlockd_sanlock
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@ -1692,6 +1695,7 @@ Optional Features:
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--enable-testing enable testing targets in the makefile
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--enable-valgrind-pool enable valgrind awareness of pools
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--disable-devmapper disable LVM2 device-mapper interaction
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--disable-aio disable asynchronous I/O
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--enable-lvmetad enable the LVM Metadata Daemon
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--enable-lvmpolld enable the LVM Polling Daemon
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--enable-lvmlockd-sanlock
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@ -3179,6 +3183,7 @@ case "$host_os" in
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LDDEPS="$LDDEPS .export.sym"
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LIB_SUFFIX=so
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DEVMAPPER=yes
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AIO=yes
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BUILD_LVMETAD=no
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BUILD_LVMPOLLD=no
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LOCKDSANLOCK=no
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@ -3198,6 +3203,7 @@ case "$host_os" in
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CLDNOWHOLEARCHIVE=
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LIB_SUFFIX=dylib
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DEVMAPPER=yes
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AIO=no
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ODIRECT=no
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DM_IOCTLS=no
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SELINUX=no
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@ -11826,6 +11832,67 @@ $as_echo "#define DEVMAPPER_SUPPORT 1" >>confdefs.h
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fi
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################################################################################
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{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether to use asynchronous I/O" >&5
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$as_echo_n "checking whether to asynchronous I/O... " >&6; }
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# Check whether --enable-aio was given.
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if test "${enable_aio+set}" = set; then :
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enableval=$enable_aio; AIO=$enableval
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fi
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{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $AIO" >&5
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$as_echo "$AIO" >&6; }
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if test "$AIO" = yes; then
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{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for io_setup in -laio" >&5
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$as_echo_n "checking for io_setup in -laio... " >&6; }
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if ${ac_cv_lib_aio_io_setup+:} false; then :
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$as_echo_n "(cached) " >&6
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else
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ac_check_lib_save_LIBS=$LIBS
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LIBS="-laio $LIBS"
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cat confdefs.h - <<_ACEOF >conftest.$ac_ext
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/* end confdefs.h. */
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/* Override any GCC internal prototype to avoid an error.
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Use char because int might match the return type of a GCC
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builtin and then its argument prototype would still apply. */
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#ifdef __cplusplus
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extern "C"
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#endif
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char io_setup ();
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int
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main ()
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{
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return io_setup ();
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;
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return 0;
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}
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_ACEOF
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if ac_fn_c_try_link "$LINENO"; then :
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ac_cv_lib_aio_io_setup=yes
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else
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ac_cv_lib_aio_io_setup=no
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fi
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rm -f core conftest.err conftest.$ac_objext \
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conftest$ac_exeext conftest.$ac_ext
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LIBS=$ac_check_lib_save_LIBS
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fi
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{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_lib_aio_io_setup" >&5
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$as_echo "$ac_cv_lib_aio_io_setup" >&6; }
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if test "x$ac_cv_lib_aio_io_setup" = xyes; then :
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$as_echo "#define AIO_SUPPORT 1" >>confdefs.h
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AIO_LIBS="-laio"
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AIO_SUPPORT=yes
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else
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AIO_LIBS=
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AIO_SUPPORT=no
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fi
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fi
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################################################################################
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{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether to build LVMetaD" >&5
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$as_echo_n "checking whether to build LVMetaD... " >&6; }
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@ -15775,6 +15842,8 @@ _ACEOF
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22
configure.in
22
configure.in
@ -39,6 +39,7 @@ case "$host_os" in
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LDDEPS="$LDDEPS .export.sym"
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LIB_SUFFIX=so
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DEVMAPPER=yes
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AIO=yes
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BUILD_LVMETAD=no
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BUILD_LVMPOLLD=no
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LOCKDSANLOCK=no
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@ -58,6 +59,7 @@ case "$host_os" in
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CLDNOWHOLEARCHIVE=
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LIB_SUFFIX=dylib
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DEVMAPPER=yes
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AIO=no
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ODIRECT=no
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DM_IOCTLS=no
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SELINUX=no
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@ -1122,6 +1124,24 @@ if test "$DEVMAPPER" = yes; then
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AC_DEFINE([DEVMAPPER_SUPPORT], 1, [Define to 1 to enable LVM2 device-mapper interaction.])
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fi
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################################################################################
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dnl -- Disable aio
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AC_MSG_CHECKING(whether to use asynchronous I/O)
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AC_ARG_ENABLE(aio,
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AC_HELP_STRING([--disable-aio],
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[disable asynchronous I/O]),
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AIO=$enableval)
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AC_MSG_RESULT($AIO)
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if test "$AIO" = yes; then
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AC_CHECK_LIB(aio, io_setup,
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[AC_DEFINE([AIO_SUPPORT], 1, [Define to 1 if aio is available.])
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AIO_LIBS="-laio"
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AIO_SUPPORT=yes],
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[AIO_LIBS=
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AIO_SUPPORT=no ])
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fi
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################################################################################
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dnl -- Build lvmetad
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AC_MSG_CHECKING(whether to build LVMetaD)
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@ -2061,9 +2081,11 @@ AC_SUBST(DEFAULT_USE_LVMETAD)
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AC_SUBST(DEFAULT_USE_LVMPOLLD)
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AC_SUBST(DEFAULT_USE_LVMLOCKD)
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AC_SUBST(DEVMAPPER)
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AC_SUBST(AIO)
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AC_SUBST(DLM_CFLAGS)
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AC_SUBST(DLM_LIBS)
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AC_SUBST(DL_LIBS)
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AC_SUBST(AIO_LIBS)
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AC_SUBST(DMEVENTD_PATH)
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AC_SUBST(DM_LIB_PATCHLEVEL)
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AC_SUBST(ELDFLAGS)
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215
doc/aio_design.txt
Normal file
215
doc/aio_design.txt
Normal file
@ -0,0 +1,215 @@
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Introducing asynchronous I/O to LVM
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===================================
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Issuing I/O asynchronously means instructing the kernel to perform specific
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I/O and return immediately without waiting for it to complete. The data
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is collected from the kernel later.
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Advantages
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----------
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A1. While waiting for the I/O to happen, the program could perform other
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operations.
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A2. When LVM is searching for its Physical Volumes, it issues a small amount of
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I/O to a large number of disks. If this was issued in parallel the overall
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runtime might be shorter while there should be little effect on the cpu time.
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A3. If more than one timeout occurs when accessing any devices, these can be
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taken in parallel, again reducing the runtime. This applies globally,
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not just while the code is searching for Physical Volumes, so reading,
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writing and committing the metadata may occasionally benefit too to some
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extent and there are probably maintenance advantages in using the same
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method of I/O throughout the main body of the code.
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A4. By introducing a simple callback function mechanism, the conversion can be
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performed largely incrementally by first refactoring and continuing to
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use synchronous I/O with the callbacks performed immediately. This allows the
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callbacks to be introduced without changing the running sequence of the code
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initially. Future projects could refactor some of the calling sites to
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simplify the code structure and even eliminate some of the nesting.
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This allows each part of what might ultimately amount to a large change to be
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introduced and tested independently.
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Disadvantages
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-------------
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D1. The resulting code may be more complex with more failure modes to
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handle. Mitigate by thorough auditing and testing, rolling out
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gradually, and offering a simple switch to revert to the old behaviour.
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D2. The linux asynchronous I/O implementation is less mature than
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its synchronous I/O implementation and might show up problems that
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depend on the version of the kernel or library used. Fixes or
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workarounds for some of these might require kernel changes. For
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example, there are suggestions that despite being supposedly async,
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there are still cases where system calls can block. There might be
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resource dependencies on other processes running on the system that make
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it unsuitable for use while any devices are suspended. Mitigation
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as for D1.
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D3. The error handling within callbacks becomes more complicated.
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However we know that existing call paths can already sometimes discard
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errors, sometimes deliberately, sometimes not, so this aspect is in need
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of a complete review anyway and the new approach will make the error
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handling more transparent. Aim initially for overall behaviour that is
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no worse than that of the existing code, then work on improving it
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later.
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D4. The work will take a few weeks to code and test. This leads to a
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significant opportunity cost when compared against other enhancements
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that could be achieved in that time. However, the proof-of-concept work
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performed while writing this design has satisfied me that the work could
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proceed and be committed incrementally as a background task.
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Observations regarding LVM's I/O Architecture
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---------------------------------------------
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H1. All device, metadata and config file I/O is constrained to pass through a
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single route in lib/device.
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H2. The first step of the analysis was to instrument this code path with
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log_debug messages. I/O is split into the following categories:
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"dev signatures",
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"PV labels",
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"VG metadata header",
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"VG metadata content",
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"extra VG metadata header",
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"extra VG metadata content",
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"LVM1 metadata",
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"pool metadata",
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"LV content",
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"logging",
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H3. A bounce buffer is used for most I/O.
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H4. Most callers finish using the supplied data before any further I/O is
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issued. The few that don't could be converted trivially to do so.
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H5. There is one stream of I/O per metadata area on each device.
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H6. Some reads fall at offsets close to immediately preceding reads, so it's
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possible to avoid these by caching one "block" per metadata area I/O stream.
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H7. Simple analysis suggests a minimum aligned read size of 8k would deliver
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immediate gains from this caching. A larger size might perform worse because
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almost all the time the extra data read would not be used, but this can be
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re-examined and tuned after the code is in place.
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Proposal
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--------
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P1. Retain the "single I/O path" but offer an asynchronous option.
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P2. Eliminate the bounce buffer in most cases by improving alignment.
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P3. Reduce the number of reads by always reading a minimum of an aligned
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8k block.
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P4. Eliminate repeated reads by caching the last block read and changing
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the lib/device interface to return a pointer to read-only data within
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this block.
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P5. Only perform these interface changes for code on the critical path
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for now by converting other code sites to use wrappers around the new
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interface.
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P6. Treat asynchronous I/O as the interface of choice and optimise only
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for this case.
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P7. Convert the callers on the critical path to pass callback functions
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to the device layer. These functions will be called later with the
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read-only data, a context pointer and a success/failure indicator.
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Where an existing function performs a sequence of I/O, this has the
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advantage of breaking up the large function into smaller ones and
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wrapping the parameters used into structures. While this might look
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rather messy and ad-hoc in the short-term, it's a first step towards
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breaking up confusingly long functions into component parts and wrapping
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the existing long parameter lists into more appropriate structures and
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refactoring these parts of the code.
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P8. Limit the resources used by the asynchronous I/O by using two
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tunable parameters, one limiting the number of outstanding I/Os issued
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and another limiting the total amount of memory used.
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P9. Provide a fallback option if asynchronous I/O is unavailable by
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sharing the code paths but issuing the I/O synchronously and calling the
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callback immediately.
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P10. Only allocate the buffer for the I/O at the point where the I/O is
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about to be issued.
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P11. If the thresholds are exceeded, add the request to a simple queue,
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and process it later after some I/O has completed.
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Future work
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-----------
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F1. Perform a complete review of the error tracking so that device
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failures are handled and reported more cleanly, extending the existing
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basic error counting mechanism.
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F2. Consider whether some of the nested callbacks can be eliminated,
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which would allow for additional simplifications.
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F3. Adjust the contents of the adhoc context structs into more logical
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arrangements and use them more widely.
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F4. Perform wider refactoring of these areas of code.
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Testing considerations
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----------------------
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T1. The changes touch code on the device path, so a thorough re-test of
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the device layer is required. The new code needs a full audit down
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through the library layer into the kernel to check that all the error
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conditions that are currently implemented (such as EAGAIN) are handled
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sensibly. (LVM's I/O layer needs to remain as solid as we can make it.)
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T2. The current test suite provides a reasonably broad range of coverage
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of this area but is far from comprehensive.
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Acceptance criteria
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-------------------
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A1. The current test suite should pass to the same extent as before the
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changes.
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A2. When all debugging and logging is disabled, strace -c must show
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improvements e.g. the expected fewer number of reads.
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A3. Running a range of commands under valgrind must not reveal any
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new leaks due to the changes.
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A4. All new coverity reports from the change must be addressed.
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A5. CPU time should be similar to that before, as the same work
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is being done overall, just in a different order.
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A6. Tests need to show improved behaviour in targetted areas. For example,
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if several devices are slow and time out, the delays should occur
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in parallel and the elapsed time should be less than before.
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Release considerations
|
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----------------------
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R1. Async I/O should be widely available and largely reliable on linux
|
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nowadays (even though parts of its interface and implementation remain a
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matter of controversy) so we should try to make its use the default
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whereever it is supported. If certain types of systems have problems we
|
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should try to detect those cases and disable it automatically there.
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R2. Because the implications of an unexpected problem in the new code
|
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could be severe for the people affected, the roll out needs to be gentle
|
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without a deadline to allow us plenty of time to gain confidence in the
|
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new code. Our own testing will only be able to cover a tiny fraction of
|
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the different setups our users have, so we need to look out for problems
|
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caused by this proactively and encourage people to test it on their own
|
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systems and report back. It must go into the tree near the start of a
|
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release cycle rather than at the end to provide time for our confidence
|
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in it to grow.
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@ -1,5 +1,8 @@
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/* include/configure.h.in. Generated from configure.in by autoheader. */
|
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|
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/* Define to 1 if aio is available. */
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#undef AIO_SUPPORT
|
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|
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/* Define to 1 to use libblkid detection of signatures when wiping. */
|
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#undef BLKID_WIPING_SUPPORT
|
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|
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|
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@ -635,6 +635,16 @@ static int _process_config(struct cmd_context *cmd)
|
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*/
|
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cmd->default_settings.udev_fallback = udev_disabled ? 1 : -1;
|
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|
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#ifdef AIO_SUPPORT
|
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cmd->use_aio = find_config_tree_bool(cmd, devices_use_aio_CFG, NULL);
|
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#else
|
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cmd->use_aio = 0;
|
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#endif
|
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if (cmd->use_aio && !dev_async_setup(cmd))
|
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cmd->use_aio = 0;
|
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|
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log_debug_io("%ssing asynchronous I/O.", cmd->use_aio ? "U" : "Not u");
|
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|
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init_retry_deactivation(find_config_tree_bool(cmd, activation_retry_deactivation_CFG, NULL));
|
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|
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init_activation_checks(find_config_tree_bool(cmd, activation_checks_CFG, NULL));
|
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|
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@ -164,6 +164,7 @@ struct cmd_context {
|
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unsigned vg_notify:1;
|
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unsigned lv_notify:1;
|
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unsigned pv_notify:1;
|
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unsigned use_aio:1;
|
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|
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/*
|
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* Filtering.
|
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|
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@ -226,6 +226,9 @@ cfg(devices_dir_CFG, "dir", devices_CFG_SECTION, CFG_ADVANCED, CFG_TYPE_STRING,
|
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cfg_array(devices_scan_CFG, "scan", devices_CFG_SECTION, CFG_ADVANCED, CFG_TYPE_STRING, "#S/dev", vsn(1, 0, 0), NULL, 0, NULL,
|
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"Directories containing device nodes to use with LVM.\n")
|
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|
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cfg(devices_use_aio_CFG, "use_aio", devices_CFG_SECTION, CFG_DEFAULT_COMMENTED, CFG_TYPE_BOOL, DEFAULT_USE_AIO, vsn(2, 2, 178), NULL, 0, NULL,
|
||||
"Use linux asynchronous I/O for parallel device access where possible.\n")
|
||||
|
||||
cfg_array(devices_loopfiles_CFG, "loopfiles", devices_CFG_SECTION, CFG_DEFAULT_UNDEFINED | CFG_UNSUPPORTED, CFG_TYPE_STRING, NULL, vsn(1, 2, 0), NULL, 0, NULL, NULL)
|
||||
|
||||
cfg(devices_obtain_device_list_from_udev_CFG, "obtain_device_list_from_udev", devices_CFG_SECTION, 0, CFG_TYPE_BOOL, DEFAULT_OBTAIN_DEVICE_LIST_FROM_UDEV, vsn(2, 2, 85), NULL, 0, NULL,
|
||||
|
||||
@ -32,6 +32,7 @@
|
||||
#define DEFAULT_SYSTEM_ID_SOURCE "none"
|
||||
#define DEFAULT_OBTAIN_DEVICE_LIST_FROM_UDEV 1
|
||||
#define DEFAULT_EXTERNAL_DEVICE_INFO_SOURCE "none"
|
||||
#define DEFAULT_USE_AIO 1
|
||||
#define DEFAULT_SYSFS_SCAN 1
|
||||
#define DEFAULT_MD_COMPONENT_DETECTION 1
|
||||
#define DEFAULT_FW_RAID_COMPONENT_DETECTION 0
|
||||
|
||||
@ -1250,6 +1250,8 @@ int dev_cache_exit(void)
|
||||
struct btree_iter *b;
|
||||
int num_open = 0;
|
||||
|
||||
dev_async_exit();
|
||||
|
||||
if (_cache.names)
|
||||
if ((num_open = _check_for_open_devices(1)) > 0)
|
||||
log_error(INTERNAL_ERROR "%d device(s) were left open and have been closed.", num_open);
|
||||
|
||||
@ -98,6 +98,85 @@ void devbufs_release(struct device *dev)
|
||||
_release_devbuf(&dev->last_extra_devbuf);
|
||||
}
|
||||
|
||||
#ifdef AIO_SUPPORT
|
||||
|
||||
# include <libaio.h>
|
||||
|
||||
static io_context_t _aio_ctx = 0;
|
||||
static int _aio_max = 128;
|
||||
|
||||
int dev_async_setup(struct cmd_context *cmd)
|
||||
{
|
||||
int r;
|
||||
|
||||
/* Already set up? */
|
||||
if (_aio_ctx)
|
||||
return 1;
|
||||
|
||||
log_debug_io("Setting up aio context for up to %d events.", _aio_max);
|
||||
|
||||
if ((r = io_setup(_aio_max, &_aio_ctx)) < 0) {
|
||||
/*
|
||||
* Possible errors:
|
||||
* ENOSYS - aio not available in current kernel
|
||||
* EAGAIN - _aio_max is too big
|
||||
* EFAULT - invalid pointer
|
||||
* EINVAL - _aio_ctx != 0 or kernel aio limits exceeded
|
||||
* ENOMEM
|
||||
*/
|
||||
log_warn("WARNING: Asynchronous I/O setup for %d events failed: %s", _aio_max, strerror(-r));
|
||||
log_warn("WARNING: Using only synchronous I/O.");
|
||||
_aio_ctx = 0;
|
||||
return 0;
|
||||
}
|
||||
|
||||
return 1;
|
||||
}
|
||||
|
||||
/* Reset aio context after fork */
|
||||
int dev_async_reset(struct cmd_context *cmd)
|
||||
{
|
||||
log_debug_io("Resetting asynchronous I/O context.");
|
||||
_aio_ctx = 0;
|
||||
|
||||
return dev_async_setup(cmd);
|
||||
}
|
||||
|
||||
void dev_async_exit(void)
|
||||
{
|
||||
int r;
|
||||
|
||||
if (!_aio_ctx)
|
||||
return;
|
||||
|
||||
log_debug_io("Destroying aio context.");
|
||||
if ((r = io_destroy(_aio_ctx)) < 0)
|
||||
/* Returns -ENOSYS if aio not in kernel or -EINVAL if _aio_ctx invalid */
|
||||
log_error("Failed to destroy asynchronous I/O context: %s", strerror(-r));
|
||||
|
||||
_aio_ctx = 0;
|
||||
}
|
||||
|
||||
#else
|
||||
|
||||
static int _aio_ctx = 0;
|
||||
|
||||
int dev_async_setup(struct cmd_context *cmd)
|
||||
{
|
||||
return 1;
|
||||
}
|
||||
|
||||
int dev_async_reset(struct cmd_context *cmd)
|
||||
{
|
||||
return 1;
|
||||
}
|
||||
|
||||
void dev_async_exit(void)
|
||||
{
|
||||
}
|
||||
|
||||
#endif /* AIO_SUPPORT */
|
||||
|
||||
/*-----------------------------------------------------------------
|
||||
* The standard io loop that keeps submitting an io until it's
|
||||
* all gone.
|
||||
|
||||
@ -202,4 +202,9 @@ void devbufs_release(struct device *dev);
|
||||
/* Return a valid device name from the alias list; NULL otherwise */
|
||||
const char *dev_name_confirmed(struct device *dev, int quiet);
|
||||
|
||||
struct cmd_context;
|
||||
int dev_async_setup(struct cmd_context *cmd);
|
||||
void dev_async_exit(void);
|
||||
int dev_async_reset(struct cmd_context *cmd);
|
||||
|
||||
#endif
|
||||
|
||||
@ -62,7 +62,8 @@ CLDFLAGS += @CLDFLAGS@
|
||||
ELDFLAGS += @ELDFLAGS@
|
||||
LDDEPS += @LDDEPS@
|
||||
LIB_SUFFIX = @LIB_SUFFIX@
|
||||
LVMINTERNAL_LIBS = -llvm-internal $(DMEVENT_LIBS) $(DAEMON_LIBS) $(SYSTEMD_LIBS) $(UDEV_LIBS) $(DL_LIBS) $(BLKID_LIBS)
|
||||
LVMINTERNAL_LIBS = -llvm-internal $(DMEVENT_LIBS) $(DAEMON_LIBS) $(SYSTEMD_LIBS) $(UDEV_LIBS) $(DL_LIBS) $(BLKID_LIBS) $(AIO_LIBS)
|
||||
AIO_LIBS = @AIO_LIBS@
|
||||
DL_LIBS = @DL_LIBS@
|
||||
RT_LIBS = @RT_LIBS@
|
||||
M_LIBS = @M_LIBS@
|
||||
|
||||
@ -116,6 +116,7 @@ int become_daemon(struct cmd_context *cmd, int skip_lvm)
|
||||
/* FIXME Clean up properly here */
|
||||
_exit(ECMD_FAILED);
|
||||
}
|
||||
dev_async_reset(cmd);
|
||||
dev_close_all();
|
||||
|
||||
return 1;
|
||||
|
||||
Loading…
Reference in New Issue
Block a user