Hi,
(With Mandriva 2006.0 and lvm2-2.01.09-1mdk)
In my recent evaluation, I found that lvm2 snapshots made on the same
original volume will all be updated the same amount of data when we make
change to the original volume.
That means they are made completely independent of each other, only
dependent on the original volume. Such design would cause serious
performance impact if we increase the number of snapshots of a volume
for the purpose of backup.
e.g. If we create one snapshot every hour and remove the old
snapshots reaching 24-hour old, we'll keep about 24 snapshots and
every write to the original volume will make 24 copies to the
snapshots before the write. It would be a big burden to the use of
the original volume.
Could anybody tell me why it's designed this way?
* My thinking:
. A reasonable design should be that a snapshot depends on one
another according to their creation time. The COW work would
reduce to mininum, only once for the whole related snapshots set
based on the same original volume.
. Deleting any snapshot among the set is still very simple and
efficient - move its blocks to the next older snapshot (indeed,
only move those blocks (identified by some ID) not existent in the
destination snapshot, drop those blocks which exist.)
. In such design, we don't need to reserve too much space for a big
number of snapshots - they totally need only about the same space
as the original volume.
* Further, conventionally snapshot is called COW, copy-on-write. But
in my point of view, it's inefficient if it's really implemented
this way. It takes one read and two writes. With the spirit of
the virtualization of logical volume management, making one write
is enough (which is efficient). What we need to do is put the old
block under the management of the newest snapshot, and make a new
block (with new content) for the original volume.
I'm newbie to this mailing list. Please feel free to comment, whether
I'm right or wrong. Thanks.
best regards,
charlie
--
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Charles Jie (紀春興) Keya Technologies (開雅科技)
(O1) +886 2 8692 6060 ext 549 Skype: charles.jie
(O2) +886 2 2938 2262 (Mobile) 0968 357 746
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