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Some major distributions are still using 'mawk' and they are not using
the latest version - we end here with hidden dependency on the latest
version of mawk (1.3.4) while i.e. Debian Lenny seems to stay with 1.3.3.
So we end with completely broken vgimportclone script on such system.
We would need to check for proper support of :space: and abort build if
it doesn't work or simplier replace [:space:] with [ \t] which seems
sufficient to make it work (as can be seen in this patch)
A better fix would be to use command line parameter override - leaving
as FIXME comment.
This patch makes t-vgimportclone.sh test passing on Lenny.
There was a bad sequence:
*) Make changes to LV layout to split images (e.g. 4-way -> 2-way/2-way)
1) vg_write, suspend_lv(original_mirror), vg_commit
2) activate_lv(newly_split_lv)
3) resume_lv(original_mirror)
Step #2 is not allowed. However, without it, the resume of the original
mirror will also resume its former sub-LVs - making it impossible to
activate the newly split LV due to the changes in layering, pointers, and
names that had already been made. Additionally, the resume or the original
brings the sub-lv's online with names that differ from the metadata on disk -
also a no-no. Thus, the split must be done in stages such that the active LVs
always reflect what is in the committed LVM metadata.
First, alter the original mirror by releasing the images. The images are made
visible and independent as an intermediate stage. (This way, we can have
consistency between LVM metadata and active LVs.) The second stage collects
the recently split LVs, deactivates them, forms them into a mirror if necessary,
and then activates them. It is a bit of a circuitous method, but it is the only
way to split a mirror from a mirror and obey these general rules:
1) Never [de]activate sub-lvs when the top-level LV is suspended
2) Avoid having active LVs that differ from the description in the LVM metadata
Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
There's a very high memory usage when calling _pv_analyse_mda_raw (e.g. while
executing pvck) that can end up with "out of memory".
_pv_analyse_mda_raw scans for metadata in the MDA, iteratively increasing the
size to scan with SECTOR_SIZE until we find a probable config section or we're
at the edge of the metadata area. However, when using a memory pool, we're also
iteratively chasing for bigger and bigger mempool chunk which can't be found
and so we're always allocating a new one, consuming more and more memory...
This patch just changes the mempool to direct memory allocation in this
problematic part of the code.
This patch adds the ability to upconvert a raid1 array - say from 2-way to
3-way. It does not yet support upconverting linear to n-way.
The 'raid' device-mapper target allows for individual components (images) of
an array to be specified for rebuild. This mechanism is used when adding
new images to the array so that the new images can be resync'ed while the
rest of the images in the array can remain 'in-sync'. (There is no
mirror-on-mirror layering required.)
~> lvconvert --splitmirrors 1 --trackchanges vg/lv
The '--trackchanges' option allows a user the ability to use an image of
a RAID1 array for the purposes of temporary read-only access. The image
can be merged back into the array at a later time and only the blocks that
have changed in the array since the split will be resync'ed. This
operation can be thought of as a partial split. The image is never completely
extracted from the array, in that the array reserves the position the device
occupied and tracks the differences between the array and the split image via
a bitmap. The image itself is rendered read-only and the name (<LV>_rimage_*)
cannot be changed. The user can complete the split (permanently splitting the
image from the array) by re-issuing the 'lvconvert' command without the
'--trackchanges' argument and specifying the '--name' argument.
~> lvconvert --splitmirrors 1 --name my_split vg/lv
Merging the tracked image back into the array is done with the '--merge'
option (included in a follow-on patch).
~> lvconvert --merge vg/lv_rimage_<n>
The internal mechanics of this are relatively simple. The 'raid' device-
mapper target allows for the specification of an empty slot in an array
via '- -'. This is what will be used if a partial activation of an array
is ever required. (It would also be possible to use 'error' targets in
place of the '- -'.) If a RAID image is found to be both read-only and
visible, then it is considered separate from the array and '- -' is used
to hold it's position in the array. So, all that needs to be done to
temporarily split an image from the array /and/ cause the kernel target's
bitmap to track (aka "mark") changes made is to make the specified image
visible and read-only. To merge the device back into the array, the image
needs to be returned to the read/write state of the top-level LV and made
invisible.
Users already have the ability to split an image from an LV of "mirror"
segtype. This patch extends that ability to LVs of "raid1" segtype.
This patch only allows a single image to be split off, however. (The
"mirror" segtype allows an arbitrary number of images to be split off.
e.g. 4-way => 3-way/linear, 2-way/2-way, linear,3-way)
of top-level LV.
We can't activate sub-lv's that are being removed from a RAID1 LV while it
is suspended. However, this is what was being used to have them show-up
so we could remove them. 'sync_local_dev_names' is a sufficient and
proper replacement and can be done after the top-level LV is resumed.
Skip decoding of DM flags when device is removed.
We currently need DM flags only for add|change events. So forking
dmsetup process for removed devices is a waste of CPU time.
Udev is already quite slow, so make it just a tiny bit faster.
Add config option to enable crc checking of VG structures.
Currently it's disabled by default.
For the internal test-suite this check it is enabled.
Note: In the case the internal error is detected, debug build with
compile option DEBUG_ENFORCE_POOL_LOCKING helps to catch the source
of the problem.
Use debug pool locking functionality. So the command could check,
whether the memory in the pool has not been modified.
For lv_postoder() instead of unlocking and locking for every changed
struct status member do it once when entering and leaving function.
(mprotect would trap each such memory access).
Currently lv_postoder() does not modify other part of vg structure
then status flags of each LV with flags that are reverted back to
its original state after function exit.
Extend vginfo cache with cached VG structure. So if the same metadata
are use, skip mda decoding in the case, the same data are in use.
This helps for operations like activation of all LVs in one VG,
where same data were decoded giving the same output result.
Patch adds 1-to-1 connection between volume_group and lvmcache_vginfo.
Move the free_vg() to vg.c and replace free_vg with release_vg
and make the _free_vg internal.
Patch is needed for sharing VG in vginfo cache so the release_vg function name
is a better fit here.
As this flag could not have been set by the current code - removing it.
Note: because of the wrong code logic this call:
lvmcache_update_vg(correct_vg, correct_vg->status & PRECOMMITTED &
(inconsistent ? INCONSISTENT_VG : 0));
had always passed '0' - now after flag removal it's passing
PRECOMMITTED flag in - this present functinal change in this patch.
To match the original functionality - 0 had to be always passed.
More testing is needed here.
(here clvmd crashed in the middle of operation),
lock is not removed from cache - here is one example:
locking/cluster_locking.c:497 Locking VG V_vg_test UN (VG) (0x6)
locking/cluster_locking.c:113 Error writing data to clvmd: Broken pipe
locking/locking.c:399 <backtrace>
locking/locking.c:461 <backtrace>
Internal error: Volume Group vg_test was not unlocked
Code should always remove lock info from lvmcache and update counters
on unlock, even if unlock fails.
Today, we use "suppress_messages" flag (set internally in init_locking fn based
on 'ignorelockingfailure() && getenv("LVM_SUPPRESS_LOCKING_FAILURE_MESSAGES")'.
This way, we can suppress high level messages like "File-based locking
initialisation failed" or "Internal cluster locking initialisation failed".
However, each locking has its own sequence of initialization steps and these
could log some errors as well. It's quite misleading for the user to see such
errors and warnings if the "--sysinit" is used (and so the ignorelockingfailure
&& LVM_SUPPRESS_LOCKING_FAILURE_MESSAGES environment variable). Errors and
warnings from these intermediary steps should be suppressed as well if requested.
This patch propagates the "suppress_messages" flag deeper into locking init
functions. I've also added these flags for other locking types for consistency,
though it's not actually used for no_locking and readonly_locking.
Last usage was removed in Petr's commit related to VG mda repair fix
where relaxed check starts to ignore inconsistencies coming from
PVs that are marked MISSING - thus removing unused variable.
Defer the test of the function return value after the string memory is released.
Otherwise in this error path the string would present memory leak.
(Thought in this case we are already out of memory...)
Implementation described in doc/lvm2-raid.txt.
Basic support includes:
- ability to create RAID 1/4/5/6 arrays
- ability to delete RAID arrays
- ability to display RAID arrays
Notable missing features (not included in this patch):
- ability to clean-up/repair failures
- ability to convert RAID segment types
- ability to monitor RAID segment types
This should be set by default! Normally we have "activation/udev_sync = 1"
in lvm.conf (example.conf.in). But if we use lvm2 without any config file
(or without a definition within '--config' option) the DEFAULT_UDEV_SYNC
is used instead. Together with verify_udev_operations=0 (when we rely on
udev fully), this can cause races as the node could be missing when needed.
(See also https://bugzilla.redhat.com/show_bug.cgi?id=723144)
Clvmd detects modifed config file before it takes lv_lock.
If the config file is changed rapidly - the change was ignored within
a seocnd ranged. This patch adds also compare of file size.
So change like some flag for 0 to 1 would pass unnoticed - but
it's quick fix for failing test suite.
FIXME: Implement inotify solution.
If MD linear device has set rounding (overload chunk size attribute),
the pvcreate command prints this warning:
/dev/md0 sysfs attr level not in expected format: linear
When some target is passing empty parameters to some dm target,
report this as an internal error to better catch some broken
table construction (some mirror conversions seem to be doing
this for now).
When an LVM mirror is up-converted (an additional image added), it creates
a temporary mirror stack. The lower-level mirror in the stack that is
created was not being activated exclusively - violating the exclusive nature
of the original mirror. We now check for exclusive activation of a mirror
before converting it, and if found, we ensure that the temporary mirror
is also exclusively activated.
Mirrors used to be created by first creating a linear device and then adding
the other images plus the log. Now mirrors are created by creating all the
images in one go and then adding the log separately. The new way ran into
the condition that cluster mirrors cannot change the log type (in the case
of creation, from core -> disk) while the mirror is not active. (It isn't
active because it is in the process of being created.) The reason this
condition is in place is because a remote node may have the mirror active, and
we don't want to alter the log underneath it.
What we really needed was a way of checking if the mirror was active remotely
but not locally, and in that case do not allow a change of the log. I've added
this check, and cluster mirrors can now be created again.
We've used udev fallback code till now to check whether udev
created/removed the entries in /dev correctly and if not,
a repair was done (giving a warning messagea about that).
This patch adds a possibility to enable this additional check
and subsequent fallback only when required (debugging purposes
mostly) and trust udev completely.
So let's disable the fallback code by default and add a new
configuration option "activation/udev_fallback".
(The original code for creating the nodes will still be used
in case the device directory that is set in lvm.conf differs
from the one that udev uses and also when activation/udev_rules
is set to 0 - otherwise we would end up with no nodes/symlinks
at all)
It's useful to keep the partial flag cached - so just move the call
for vg_mark_partil_lvs() into import_vg_from_config_tree() so it gets
evaluated before it goes through the lvmcache.
This patch should not present any functional change.
Note: It is rather temporal solution - proper place is probably inside the
'read' call back - but needs some more discussion.
For now using this minor hack.
As the ACTIVATE_EXCL could be set only in clvmd code - there is no
use for this test in lv_add_mirrors() function only called from
tools context.
FIXME: Add cluster test case for this.
To avoid modification of 'read-only' volume group structure
add a new structure to pass local data around the code for LV
activation.
As origin_only is one such flag - replace this parameter with new
struct lv_activate_opts.
More parameters might eventually become part of lv_activate_opts.
This was missing in liblvm and it caused all udev-related operations to
not take effect when using liblvm, e.g. obtaining the list of devices from udev
db instead of scanning the whole /dev which also recreated the .cache as a side
effect. This was also the case with udisks-lvm-pv-export prober which is run
from within udev rules whenever the CHANGE event is fired.
and use this for the LVM critical section logic. Also report an error if
code tries to load a table while any device is known to be in the
suspended state.
(If the variety of problems these changes are showing up can't be fixed
before the next release, the error messages can be reduced to debug
level.)
are affected by the move. (Currently it's possible for I/O to become
trapped between suspended devices amongst other problems.
The current fix was selected so as to minimise the testing surface. I
hope eventually to replace it with a cleaner one that extends the
deptree code.
Some lvconvert scenarios still suffer from related problems.
Patch adds check for stripe not only in direct
LV segment but also in mirror image segment.
This prevents bugs like:
# lvcreate -i2 -l10 -n lv vg_test
# lvconvert -m1 -i1 vg_test/lv
# lvreduce -f -l1 vg_test/lv
WARNING: Reducing active logical volume to 4.00 MiB
THIS MAY DESTROY YOUR DATA (filesystem etc.)
Reducing logical volume lv to 4.00 MiB
Segment extent reduction 9 not divisible by #stripes 2
Logical volume lv successfully resized
# lvremove -f vg_test
Segment extent reduction 1 not divisible by #stripes 2
LV segment lv:0-4294967295 is incorrectly listed as being used by LV lv_mimage_0
Internal error: LV segments corrupted in lv_mimage_0.
Currently some operation with striped mirrors lead
to corrupted metadata, this patch just add detection of such
situation.
Example:
# lvcreate -i2 -l10 -n lvs vg_test
# lvconvert -m1 vg_test/lvs
# lvreduce -f -l1 vg_test/lvs
Reducing logical volume lvs to 4.00 MiB
Segment extent reduction 9not divisible by #stripes 2
Logical volume lvs successfully resized
# lvremove vg_test/lvs
Segment extent reduction 1not divisible by #stripes 2
LV segment lvs:0-4294967295 is incorrectly listed as being used by LV lvs_mimage_0
Internal error: LV segments corrupted in lvs_mimage_0.
We should never remove more extents than requested by user,
so round up to next stripe boundary during lvreduce.
Also this fixes round to zero sized LV bug:
# lvcreate -i2 -I 64k -l10 -n lvs vg_test
# lvreduce -f -l1 vg_test/lvs
Rounding size (1 extents) down to stripe boundary size for segment (0 extents)
WARNING: Reducing active logical volume to 0
THIS MAY DESTROY YOUR DATA (filesystem etc.)
Reducing logical volume lvs to 0
Failed to suspend lvs
There's a possibility someone will use the '/' in the hostname. Since we
generate a temporary file name (path) including the hostname, any '/' would
be ambiguous.
We can always set such hostname using 'sethostname' from unistd.h. But the
'hostname' command already includes the check and removes the '/' char.
However, some old versions still allow that.
See: https://bugzilla.redhat.com/show_bug.cgi?id=711445.
Since this is only a temporary name and the possibility of this error is
quite negligible, we don't need any complex escape sequence here, just a
simple char replace.
Also, add a new 'obtain_device_list_from_udev' setting to lvm.conf with which
we can turn this feature on or off if needed.
If set, the cache of block device nodes with all associated symlinks
will be constructed out of the existing udev database content.
This avoids using and opening any inapplicable non-block devices or
subdirectories found in the device directory. This setting is applied
to udev-managed device directory only, other directories will be scanned
fully. LVM2 needs to be compiled with udev support for this setting to
take effect. N.B. Any device node or symlink not managed by udev in
udev directory will be ignored with this setting on.
Related to rhbz 697959.
This test fails prior to these two commits:
commit af112eb2c9
Author: Zdenek Kabelac <zkabelac@redhat.com>
Date: Thu Apr 21 13:15:26 2011 +0000
Skip check for NULL before dm_free
dm_free makes this test itself.
commit 91419c3e86
Author: Zdenek Kabelac <zkabelac@redhat.com>
Date: Thu Apr 21 13:13:40 2011 +0000
Fix use of released vgname and vgid
Avoid using of already released memory when duplicated MDA is found.
As get_pv_from_vg_by_id() may call lvmcache_label_scan() use the local copy
of the vgname and vgid on the stack as vginfo may dissapear and code was
then accessing garbage in memory.
i.e. pvs /dev/loop0
(when /dev/loop0 and /dev/loop1 has same MDA content)
Invalid read of size 1
at 0x523C986: dm_hash_lookup (hash.c:325)
by 0x440C8C: vginfo_from_vgname (lvmcache.c:399)
by 0x4605C0: _create_vg_text_instance (format-text.c:1882)
by 0x46140D: _text_create_text_instance (format-text.c:2243)
by 0x47EB49: _vg_read (metadata.c:2887)
by 0x47FBD8: vg_read_internal (metadata.c:3231)
by 0x477594: get_pv_from_vg_by_id (metadata.c:344)
by 0x45F07A: _get_pv_if_in_vg (format-text.c:1400)
by 0x45F0B9: _populate_pv_fields (format-text.c:1414)
by 0x45F40F: _text_pv_read (format-text.c:1493)
by 0x480431: _pv_read (metadata.c:3500)
by 0x4802B2: pv_read (metadata.c:3462)
Address 0x652ab80 is 0 bytes inside a block of size 4 free'd
at 0x4C2756E: free (vg_replace_malloc.c:366)
by 0x442277: _free_vginfo (lvmcache.c:963)
by 0x44235E: _drop_vginfo (lvmcache.c:992)
by 0x442B23: _lvmcache_update_vgname (lvmcache.c:1165)
by 0x443449: lvmcache_update_vgname_and_id (lvmcache.c:1358)
by 0x443C07: lvmcache_add (lvmcache.c:1492)
by 0x46588C: _text_read (text_label.c:271)
by 0x466A65: label_read (label.c:289)
by 0x4413FC: lvmcache_label_scan (lvmcache.c:635)
by 0x4605AD: _create_vg_text_instance (format-text.c:1881)
by 0x46140D: _text_create_text_instance (format-text.c:2243)
by 0x47EB49: _vg_read (metadata.c:2887)
Add testing script
pv_manip.c to properly account for case when pe_start=0 and the first
physical extent is to be released (currently skip the first extent to
avoid discarding the PV label).
dm_pool_free incorrectly. This check-in fixes that incorrect usage.
I've also added a WHATS_NEW line to reflect the changes I made to allow
lv_extend to operate on 0 length intrinsically layered LVs (i.e mirrors
and RAID). I forgot that in the last commit.
Instead of regenerating config tree and parsing same data again,
check whether export_vg_to_buffer does not produce same string as
the one already cached - in this case keep it, otherwise throw cached
content away.
For the code simplicity calling _free_cached_vgmetadata() with
vgmetadata == NULL as the function handles this itself.
Note: sometimes export_vg_to_buffer() generates almost the same data
with just different time stamp, but for the patch simplicity,
data are reparsed in this case.
This patch currently helps for vgrefresh.
Avoid locking sum testing with valgrind compilation.
Make memory unaccessible in the valgrind for dm_pool_abadon_object.
Valgrind hinting should not be needed in _free_chunk for dm_free.
'a small step' towards cleaner shutdown sequence.
Normally clvmd doens't care about unreleased memory on exit -
but for valgrind testing it's better to have them cleaned all.
So - few things are left on exit path - this patch starts to remove
just some of them.
1. lvm_thread_fs is made as a thread which could be joined on exit()
2. memory allocated to local_clien_head list is released.
(this part is somewhat more complex if the proper reaction is
needed - and as it requires some heavier code moving - it will
be resolved later.
Could be reached via few of our lvm2 test cases:
==11501== Invalid read of size 8
==11501== at 0x49B2E0: _area_length (import-extents.c:204)
==11501== by 0x49B40C: _read_linear (import-extents.c:222)
==11501== by 0x49B952: _build_segments (import-extents.c:323)
==11501== by 0x49B9A0: _build_all_segments (import-extents.c:334)
==11501== by 0x49BB4C: import_extents (import-extents.c:364)
==11501== by 0x497655: _format1_vg_read (format1.c:217)
==11501== by 0x47E43E: _vg_read (metadata.c:2901)
cut from t-vgcvgbackup-usage.sh
--
pvcreate -M1 $(cat DEVICES)
vgcreate -M1 -c n $vg $(cat DEVICES)
lvcreate -l1 -n $lv1 $vg $dev1
--
Idea of the fix is rather defensive - to allocate one extra element
to 'map' array which is then used in _area_length() - where the
loop checks, whether next map entry is continuous.
By placing there always one extra zero entry -
we fix the read of unallocated memory, and we make sure the data would
not make a continous block.
FIXME: there could be a problem if some special broken lvm1 data would be imported.
As the format1 is currently not really used - leave it for future fix
and use this small hotfix for now.
Invalid primary_vginfo was supposed to move all its lvmcache_infos to
orphan_vginfo - however it has called _drop_vginfo() inside the loop
that released primary_vginfo itself - thus made the loop using released
memory.
Use _vginfo_detach_info() instead and call _drop_vginfo after
th loop is finished.
Valgrind trace it should fix:
Invalid read of size 8
at 0x41E960: _lvmcache_update_vgname (lvmcache.c:1229)
by 0x41EF86: lvmcache_update_vgname_and_id (lvmcache.c:1360)
by 0x441393: _text_read (text_label.c:329)
by 0x442221: label_read (label.c:289)
by 0x41CF92: lvmcache_label_scan (lvmcache.c:635)
by 0x45B303: _vg_read_by_vgid (metadata.c:3342)
by 0x45B4A6: lv_from_lvid (metadata.c:3381)
by 0x41B555: lv_activation_filter (activate.c:1346)
by 0x415868: do_activate_lv (lvm-functions.c:343)
by 0x415E8C: do_lock_lv (lvm-functions.c:532)
by 0x40FD5F: do_command (clvmd-command.c:120)
by 0x413D7B: process_local_command (clvmd.c:1686)
Address 0x63eba10 is 16 bytes inside a block of size 160 free'd
at 0x4C2756E: free (vg_replace_malloc.c:366)
by 0x41DE70: _free_vginfo (lvmcache.c:980)
by 0x41DEDA: _drop_vginfo (lvmcache.c:998)
by 0x41E854: _lvmcache_update_vgname (lvmcache.c:1238)
by 0x41EF86: lvmcache_update_vgname_and_id (lvmcache.c:1360)
by 0x441393: _text_read (text_label.c:329)
by 0x442221: label_read (label.c:289)
by 0x41CF92: lvmcache_label_scan (lvmcache.c:635)
by 0x45B303: _vg_read_by_vgid (metadata.c:3342)
by 0x45B4A6: lv_from_lvid (metadata.c:3381)
by 0x41B555: lv_activation_filter (activate.c:1346)
by 0x415868: do_activate_lv (lvm-functions.c:343)
problematic line:
dm_list_iterate_items_safe(info2, info3, &primary_vginfo->infos)
Fix 2 more functions sending cluster messages to avoid passing uninitilised bytes
and compensate 1 extra byte attached to the message from the clvm_header.args[1]
member variable.
LVM doesn't behave correctly if running as non-root user,
there is warning when it detects it.
Despite this, it produces many error messages, saying nothing.
See https://bugzilla.redhat.com/show_bug.cgi?id=620571
This patch fixes two things:
1) Removes eror message from device_is_usable() which has no
information value anyway (real warning is printed inside it).
2) it fixes device-mapper initialization, if we support
core dm module autoload and device node is present, it should
fail early and not try recreate existing and correct node.
(non-root == permission denied here)
N.B. In future code should support user roles, some more
drastic checks in code are probably contraproductive now.
Attach \0 for proper char* display - otherwise somewhat random message could
be displayed in debug more and read of unpredictable read of uninitilized
memory values could happen.
As code uses strncpy(system_id, NAME_LEN) and doesn't set '\0'
Fix it by always allocating NAME_LEN + 1 buffer size and with zalloc
we always get '\0' as the last byte.
This bug may trigger some unexpected behavior of the string operation
code - depends on the pool allocator.
FIXME: refactor this code to alloc_vg.
We have 3 components and traling '\0' so allocate proper room for all of them.
Problem was nicely hidden by allocation from pool and allocation aligment
offset - so to trigger real problem with this one is actually hard.
Return value of readlink limits valid string size.
Characters after returned size present some garbage to printf.
Fix it by placing '\0' on the return size value.
This is necessary for proper format instance ref_count support. We iterate
over vg->pvs and vg->removed_pvs list and the ref_count is decremented and
then it is destroyed if not referenced anymore.
Since format instances will use own memory pool, it's necessary to properly
deallocate it. For now, only fid is deallocated. The PV structure itself
still uses cmd mempool mostly, but anytime we'd like to add a mempool
in the struct physical_volume, we can just rename this fn to free_pv and
add the code (like we have free_vg fn for VGs).
This is essential for proper format instance ref_count support. We must
use these functions to set the fid everywhere from now on, even the NULL
value!
We'd like to use the fid mempool for text_context that is stored
in the instance (we used cmd mempool before, so the order of
initialisation was not a matter, but now it is since we need to
create the fid mempool first which happens in create_instance fn).
The text_context initialisation is not needed anywhere outside the
create_instance fn so move it there.
Format instances can be created anytime on demand and it contains
metadata area information mostly (at least for now, but in the future,
we may store more things here to update/edit in a PV/VG). In case we
have lots of metadata areas, memory consumption will rise. Using cmd
context mempool is not quite optimal here because it is destroyed too
late. So let's use a separate mempool for format instances.
Reference counting is used because fids could be shared, e.g. each PV
has either a PV-based fid or VG-based fid. If it's VG-based, each PV has
a shared fid with the VG - a reference to VG's fid.
