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I'm exploring using LVM snapshot's to off site incremental archives from a snapshot "master" file system. In essence: simply copy across only the files on the "master" that have changed since the last incremental copy to the "archive". Then snapshot the "archive" to retain the incremental.

I am a bit puzzled as to the block usage behaviour of the archive's own incremental snapshot.

I'm expecting that LVM is not smart enough to know that the "file block" is actually unchanged, and the a new copy will be allocated and written for the fresh "archive" file system.

Can anyone confirm this, or point me to a document/page that gives some hints?

BTW: the OS hard disk cache, hard disk physical cache and hard disk itself also doesn't need to do any actual "disk writes" as the "disk block" likewise is unnecessary. Any pointers to discussion of this style of optimisation would also be ineresting.

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If I understand correctly you need to know some thing known as rsync it does what you are asking.

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rsync is a (file level) method could that be used to actually transport the data from the master to the remote archive. But I'm more interested in how the data is stored in the remote archive. In particular I want to be able to use a LVM snapshot to be able to restore historic systems. Eg Last week, last month, year end etc. Ideally LVM might be able to be used to conserve disk space for files/blocks that are unchanged, or change very little. Then I can selectively remove archive snapshots as we determine them to be surplus. – NevilleDNZ Jan 5 '11 at 7:00
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