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dm snapshot: add optional discard support features

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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 <[email protected]>
Signed-off-by: Mike Snitzer <[email protected]>
This commit is contained in:
Mike Snitzer 2019-06-19 17:05:54 -04:00
commit 2e6023850e
2 changed files with 181 additions and 21 deletions
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16
Documentation/device-mapper/snapshot.txt
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@ -31,6 +31,7 @@ its visible content unchanged, at least until the <COW device> fills up.
*) snapshot <origin> <COW device> <persistent?> <chunksize>
[<# feature args> [<arg>]*]
A snapshot of the <origin> block device is created. Changed chunks of
<chunksize> sectors will be stored on the <COW device>. Writes will
@ -53,8 +54,23 @@ When loading or unloading the snapshot target, the corresponding
snapshot-origin or snapshot-merge target must be suspended. A failure to
suspend the origin target could result in data corruption.
Optional features:
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.
* snapshot-merge <origin> <COW device> <persistent> <chunksize>
[<# feature args> [<arg>]*]
takes the same table arguments as the snapshot target except it only
works with persistent snapshots. This target assumes the role of the