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commit 2e6023850e177dbaca21498ada04c5a5ac93f812 upstream.
discard_zeroes_cow - a discard issued to the snapshot device that maps
to entire chunks to will zero the corresponding exception(s) in the
snapshot's exception store.
discard_passdown_origin - a discard to the snapshot device is passed down
to the snapshot-origin's underlying device. This doesn't cause copy-out
to the snapshot exception store because the snapshot-origin target is
bypassed.
The discard_passdown_origin feature depends on the discard_zeroes_cow
feature being enabled.
When these 2 features are enabled they allow a temporarily read-only
device that has completely exhausted its free space to recover space.
To do so dm-snapshot provides temporary buffer to accommodate writes
that the temporarily read-only device cannot handle yet. Once the upper
layer frees space (e.g. fstrim to XFS) the discards issued to the
dm-snapshot target will be issued to underlying read-only device whose
free space was exhausted. In addition those discards will also cause
zeroes to be written to the snapshot exception store if corresponding
exceptions exist. If the underlying origin device provides
deduplication for zero blocks then if/when the snapshot is merged backed
to the origin those blocks will become unused. Once the origin has
gained adequate space, merging the snapshot back to the thinly
provisioned device will permit continued use of that device without the
temporary space provided by the snapshot.
Requested-by: John Dorminy <jdorminy@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
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Add a note to snapshot.txt that the origin target must be suspended when
loading or unloading the snapshot target.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
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Commit 76c44f6d80 introduced the possibly for "Overflow" to be reported
by the snapshot device's status. Older userspace (e.g. lvm2) does not
handle the "Overflow" status response.
Fix this incompatibility by requiring newer userspace code, that can
cope with "Overflow", request the persistent store with overflow support
by using "PO" (Persistent with Overflow) for the snapshot store type.
Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Fixes: 76c44f6d80 ("dm snapshot: don't invalidate on-disk image on snapshot write overflow")
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
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Update Documentation/device-mapper/snapshot.txt to cover "How to
determine when a snapshot has finished merging".
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
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The snapshot-merge target allows a snapshot to be merged back into the
snapshot's origin device.
One anticipated use of snapshot merging is the rollback of filesystems
to back out problematic system upgrades.
This patch adds snapshot-merge target management to both
dm_snapshot_init() and dm_snapshot_exit(). As an initial place-holder,
snapshot-merge is identical to the snapshot target. Documentation is
provided.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
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I've recently added this documentation, Alasdair gave some corrections, and
here are some further corrections on top of his work (partly style issue,
partly a technical error due to different past experience, partly a note
which I've added - i.e. transient snapshots are lighter).
Cc: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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I've recently discovered the real functionality of device-mapper snapshots,
and since they are not well known, I've decided to write some docs for
them.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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Mike Snitzer
Mikulas Patocka
Paolo 'Blaisorblade' Giarrusso