Jiann-Ming Su wrote:
On 8/22/07, Scott Ehrlich <scott@xxxxxxx> wrote:
Having snapshot technology is great. Who else supports it?
Roll your own:
http://www.mikerubel.org/computers/rsync_snapshots/
http://www.dirvish.org/
those aren't even close to the same thing, more like traditional
backups, rsync or dirvish make copies of the original data.
'snapshotting' in this context refers to 'copy on write' technology.
taking a snapshot is a very quick process, usually only a couple
seconds, and no data actually gets copied. when the file system is
updated while a snapshot is active, THEN the block(s) behind the update
are copied to new space, so that there are now two versions of that
block, original (snapshot) and new (current). If the snapshot is
deleted, these copied blocks are released back into the free pool. if
the system is rolled back to the snapshot, the 'new' blocks are released.
A classic use of a snapshot is to make a point-in-time backup of a live
system to tape or an external near line disk storage system... you
snapshot the file system, let the applications keep running on the live
system, and backup the snapshot to your archival storage, then drop the
snapshot. Another use is as a checkpoint before doing something
risky, like a system update. snapshot the system, install the updates
and test them. if there's a problem, rollback to the snapshot.
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