From: Paolo 'Blaisorblade' Giarrusso 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 Signed-off-by: Paolo 'Blaisorblade' Giarrusso Signed-off-by: Andrew Morton --- Documentation/device-mapper/snapshot.txt | 5 +++-- 1 files changed, 3 insertions(+), 2 deletions(-) diff -puN Documentation/device-mapper/snapshot.txt~fix-dm-snapshot-tutorial-in-documentation Documentation/device-mapper/snapshot.txt --- 25/Documentation/device-mapper/snapshot.txt~fix-dm-snapshot-tutorial-in-documentation Tue Oct 11 14:30:12 2005 +++ 25-akpm/Documentation/device-mapper/snapshot.txt Tue Oct 11 14:30:12 2005 @@ -19,7 +19,6 @@ There are two dm targets available: snap *) snapshot-origin which will normally have one or more snapshots based on it. -You must create the snapshot-origin device before you can create snapshots. Reads will be mapped directly to the backing device. For each write, the original data will be saved in the of each snapshot to keep its visible content unchanged, at least until the fills up. @@ -27,7 +26,7 @@ its visible content unchanged, at least *) snapshot -A snapshot is created of the block device. Changed chunks of +A snapshot of the block device is created. Changed chunks of sectors will be stored on the . Writes will only go to the . Reads will come from the or from for unchanged data. will often be @@ -37,6 +36,8 @@ the amount of free space and expand the is P (Persistent) or N (Not persistent - will not survive after reboot). +The difference is that for transient snapshots less metadata must be +saved on disk - they can be kept in memory by the kernel. How this is used by LVM2 _