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There is a problem with the way mirrors have been designed to handle
failures that is resulting in stuck LVM processes and hung I/O. When
mirrors encounter a write failure, they block I/O and notify userspace
to reconfigure the mirror to remove failed devices. This process is
open to a couple races:
1) Any LVM process other than the one that is meant to deal with the
mirror failure can attempt to read the mirror, fail, and block other
LVM commands (including the repair command) from proceeding due to
holding a lock on the volume group.
2) If there are multiple mirrors that suffer a failure in the same
volume group, a repair can block while attempting to read the LVM
label from one mirror while trying to repair the other.
Mitigation of these races has been attempted by disallowing label reading
of mirrors that are either suspended or are indicated as blocking by
the kernel. While this has closed the window of opportunity for hitting
the above problems considerably, it hasn't closed it completely. This is
because it is still possible to start an LVM command, read the status of
the mirror as healthy, and then perform the read for the label at the
moment after a the failure is discovered by the kernel.
I can see two solutions to this problem:
1) Allow users to configure whether mirrors can be candidates for LVM
labels (i.e. whether PVs can be created on mirror LVs). If the user
chooses to allow label scanning of mirror LVs, it will be at the expense
of a possible hang in I/O or LVM processes.
2) Instrument a way to allow asynchronous label reading - allowing
blocked label reads to be ignored while continuing to process the LVM
command. This would action would allow LVM commands to continue even
though they would have otherwise blocked trying to read a mirror. They
can then release their lock and allow a repair command to commence. In
the event of #2 above, the repair command already in progress can continue
and repair the failed mirror.
This patch brings solution #1. If solution #2 is developed later on, the
configuration option created in #1 can be negated - allowing mirrors to
be scanned for labels by default once again.
Add allocation/thin_pool_chunk_size_calculation lvm.conf
option to select a method for calculating thin pool chunk
sizes and define two possible values - "default" and "performance".
lvmetad is not yet supported in clustered environment so
disable it automatically if using lvmconf --enable-cluster
and reset it to default value if using lvmconf --disable-cluster.
Also, add a few comments in lvm.conf about locking_type vs. use_lvmetad
if setting it for clustered environment.
When both the '-i' and '-m' arguments are specified on the command
line, use the "raid10" segment type. This way, the native RAID10
personality is used through dm-raid rather than layering a mirror
on striped LVs. If the old behavior is desired, the '--type'
argument to use would be "mirror" rather than "raid10".
Add new configure lvm.conf options for binaries thin_repair
and thin_dump.
Those are part of device-mapper-persistent-data package
and will be used for recovery of thin_pool.
The activation/auto_set_activation_skip enables/disables automatic
adding of the ACTIVATION_SKIP LV flag. By default thin snapshots
are flagged to be skipped during activation.
And by default, the auto_set_activation_skip is enabled.
