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Re: [linux-lvm] Revised petition WAS: LVM in stock kernel!?





I've been able to succesully mirror lvs using the raid0 implmentation along with LVM since I started using it.
As for the snapshot backup, in a nutshell, you simply mark a stale mirror (or break the mirror temporarily),
and set ADSM to back up the logical volume directly (works fine, and quickly) Again, I've been using the Linux
adsm client for almost 2 years now (pays to be an insider with IBM hehehe), and once LVM came out I immediately
began testing LV backups. True, a data restore requires a complete rebuild of the mirror, but the client will
get it. In some situations this is not optimal (e.g. if you have a filesystem using the LV), but in the case
of raw LV access (e.g. Sybase/Oracle/DB2), it works well.

This is not quite the same concept as snapshot filesystems, although you
can certainly use it for reliable backups (quiescent filesystems!). With a spare
disk you can even use lv and a floating mirror disk to help back up several mirrored
devices - attach it to one mirror, sync up, detach and backup, attach to another
mirror, sync, etc.


Snapshots (at least the concept that Veritas has described to the world) let you
freeze the state of a filesystem as a read-only image while the filesystem itself
remains live, but without breaking any mirrors and leaving yourself temporarily in
an unmirrored state. All you really need is to be able to freeze a pointer to the
metadata, and set the FS into a state such that no disk block can be modified,
instead it's always copied on write. Requires suitable filesystem support, of
course. Those extra blocks are released only when you're done with the snapshot.
Snapshots can live for as long as you want, and there can be several of them
active, as long as you have disk space that is.




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