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The new label_scan() function reads a large buffer of data
from the start of the disk, and saves it so that multiple
structs can be read from it. Previously, only the label_header
was read from this buffer, and the code which needed data
structures that immediately followed the label_header would
read those from disk separately. This created a large
number of small, unnecessary disk reads.
In each place that the two read paths (label_scan and vg_read)
need to read data from disk, first check if that data is
already available from the label_read_data buffer, and if
so just copy it from the buffer instead of reading from disk.
Code changes
------------
- passing the label_read_data struct down through
both read paths to make it available.
- before every disk read, first check if the location
and size of the desired piece of data exists fully
in the label_read_data buffer, and if so copy it
from there. Otherwise, use the existing code to
read the data from disk.
- adding some log_error messages on existing error paths
that were already being updated for the reasons above.
- using similar naming for parallel functions on the two
parallel read paths that are being updated above.
label_scan path calls:
read_metadata_location_summary, text_read_metadata_summary
vg_read path calls:
read_metadata_location_vg, text_read_metadata_file
Previously, those functions were named:
label_scan path calls:
vgname_from_mda, text_vgsummary_import
vg_read path calls:
_find_vg_rlocn, text_vg_import_fd
I/O changes
-----------
In the label_scan path, the following data is either copied
from label_read_data or read from disk for each PV:
- label_header and pv_header
- mda_header (in _raw_read_mda_header)
- vg metadata name (in read_metadata_location_summary)
- vg metadata (in config_file_read_fd)
Total of 4 reads per PV in the label_scan path.
In the vg_read path, the following data is either copied from
label_read_data or read from disk for each PV:
- mda_header (in _raw_read_mda_header)
- vg metadata name (in read_metadata_location_vg)
- vg metadata (in config_file_read_fd)
Total of 3 reads per PV in the vg_read path.
For a common read/reporting command, each PV will be:
- read by the command's initial lvmcache_label_scan()
- read by lvmcache_label_rescan_vg() at the start of vg_read()
- read by vg_read()
Previously, this would cause 11 synchronous disk reads per PV:
4 from lvmcache_label_scan(), 4 from lvmcache_label_rescan_vg()
and 3 from vg_read().
With this commit's optimization, there are now 2 async disk reads
per PV: 1 from lvmcache_label_scan() and 1 from
lvmcache_label_rescan_vg().
When a second mda is used on a PV, it is located at the
end of the PV. This second mda and copy of metadata will
not be found in the label_read_data buffer, and will always
require separate disk reads.
Refactor the recent metadata-reading optimisation patches.
Remove the recently-added cache fields from struct labeller
and struct format_instance.
Instead, introduce struct lvmcache_vgsummary to wrap the VG information
that lvmcache holds and add the metadata size and checksum to it.
Allow this VG summary information to be looked up by metadata size +
checksum. Adjust the debug log messages to make it clear when this
shortcut has been successful.
(This changes the optimisation slightly, and might be extendable
further.)
Add struct cached_vg_fmtdata to format-specific vg_read calls to
preserve state alongside the VG across separate calls and indicate
if the details supplied match, avoiding the need to read and
process the VG metadata again.
Use similar logic as with text_vg_import_fd() and avoid repeated
parsing of same mda and its config tree for vgname_from_mda().
Remember last parsed vgname, vgid and creation_host in labeller
structure and if the metadata have the same size and checksum,
return this stored info.
TODO: The reuse of labeller struct is not ideal, some lvmcache API for
this functionality would be nicer.
The list of strings is used quite frequently and we'd like to reuse
this simple structure for report selection support too. Make it part
of libdevmapper for general reuse throughout the code.
This also simplifies the LVM code a bit since we don't need to
include and manage lvm-types.h anymore (the string list was the
only structure defined there).
Just to prevent accidental and improper use when reading the layout
from disk because of the already existing disk_areas_xl[0] lists
that are variable in size. We can read pv_header_extension only
after we know exactly where the lists end...
New tools with PV header extension support will read the extension
if it exists and it's not an error if it does not exist (so old PVs
will still work seamlessly with new tools).
Old tools without PV header extension support will just ignore any
extension.
As for the Embedding Area location information (its start and size),
there are actually two places where this is stored:
- PV header extension
- VG metadata
The VG metadata contains a copy of what's written in the PV header
extension about the Embedding Area location (NULL value is not copied):
physical_volumes {
pv0 {
id = "AkSSRf-difg-fCCZ-NjAN-qP49-1zzg-S0Fd4T"
device = "/dev/sda" # Hint only
status = ["ALLOCATABLE"]
flags = []
dev_size = 262144 # 128 Megabytes
pe_start = 67584
pe_count = 23 # 92 Megabytes
ea_start = 2048
ea_size = 65536 # 32 Megabytes
}
}
The new metadata fields are "ea_start" and "ea_size".
This is mostly useful when restoring the PV by using existing
metadata backups (e.g. pvcreate --restorefile ...).
New tools does not require these two fields to exist in VG metadata,
they're not compulsory. Therefore, reading old VG metadata which doesn't
contain any Embedding Area information will not end up with any kind
of error but only a debug message that the ea_start and ea_size values
were not found.
Old tools just ignore these extra fields in VG metadata.
PV header extension comes just beyond the existing PV header base:
PV header base (existing):
- uuid
- device size
- null-terminated list of Data Areas
- null-terminater list of MetaData Areas
PV header extension:
- extension version
- flags
- null-terminated list of Embedding Areas
This patch also adds "eas" (Embedding Areas) list to lvmcache (lvmcache_info)
and it also adds support for common operations on the list (just like for
already existing "das" - Data Areas list):
- lvmcache_add_ea
- lvmcache_update_eas
- lvmcache_foreach_ea
- lvmcache_del_eas
Also, add ea_start and ea_size to struct physical_volume for processing
PV Embedding Area location throughout the code (currently only one
Embedding Area is supported, though the definition on disk allows for
more if needed in the future...).
Also, define FMT_EAS format flag to mark that the format actually
supports Embedding Areas (currently format-text only).
We'd like to pass in mda_header to vgname_from_mda(). In order to
do this, we need to call raw_read_mda_header() from text_label.c,
_text_read(), which gets called from the label_read() path, and
peers into the metadata and update vginfo cache. We should check
the disable bit here, and if set, not peer into the vg metadata,
thus reducing the I/O to disk.
In the process, move vgname_from_mda() to layout.h, since the fn
only gets called from format_text code, and we need the mda_header
definition from the private layout.h.
Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Adding a flag to the 'rlocn' structure in the mda header of the
text format allows us to flip a bit to ignore an area on disk that
stores the metadata via the text format specific mda_header.
This patch defines the flag and access functions to manage the flag.
Other patches will manage the ignore on a format-independent basis,
by using a flag in the metadata_area structure.
Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Future patches will make use of a specific flag in the on-disk 'raw_locn'
structure to enable/disable metadata areas, and facilitate metadata
balancing.
Note that 'filler' is always set to '0' (see add_mda() - memset),
so use of this area as a non-zero flags field is a safe way to
provide future code features.
Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Fix some memory leaks in error paths found by coverity.
Use C99 struct initialisers.
Move DEFS into configure.h.
Clean-ups to remove miscellaneous compiler warnings.
