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[linux-lvm] Snapshot questions



Hi,
I have been looking into doing snapshots with LVM and i have found the documentation is very light on how they work. Sorry for all the questions, they are probably very easy and i realize its a lot but the documentation says almost nothing about this and i would really like to know. Even just pointing me to something that answers this would be nice. So far all i have found is tldp.org and that has about 1 page on snapshots and there are about 100 other sites listing the same text (almost word for word) So I'm going to ask my questions here and hopefully get an idea on how it all works.

First what exactly is the size of a snapshot used for? Is the the total difference (changed data) between the snapshot and the active LV that it was created against? What happens if i have multiple snapshots? are the snapshots changed data shared, so if i have LV1, make snap1 from LV1, then change LV1 it uses space in snap1 correct? what if i then add snap2 created from LV1, does modifying LV1 remove free space from snap1 and snap2, or just snap2?

If snapshots only use the space that has been changed between it and the LV it was created from then does that imply that a new snapshot uses zero space on the VG (if i made its size 1GB) and grow as data is changed, or does it use 1GB of space on the VG immediately, basically i want to know if the specified size of the snapshots is allowed to sum to more then the space on the VG (so if the VG is 200GB, and i put a 100GB LV on it, am i limited to 10 10GB snaps (to sum to the 100GB), or can i put 10 50GB snaps on it (which would probably only have a very small percent of usage) and let then get auto-deleted as space runs out?

I saw that snapshots are deleted when the LV is full, does this mean when the snapshot differers by more then the changes made since it was created? if so what snapshot gets deleted? is it just the one that ran out of space? what if there was a snapshot that came before it that has not run out of space?

What about fragmentation, does LVM handle 50 snapshots of a single VG (against a single LV), or will that cause massive fragmentation? does LVM have problems with high numbers of snapshots, or is 50+ considered normal?

And for the last question, I'm building a backup server, right now I'm taking a RAID, putting LVM on it, and I'm thinking about taking weekly snapshots of the backup data to allow me to rollback (for as far back as the disk can take me). does this sound like a good idea, or should i try something else? what is bad about this plan if its a bad idea? do you have any other recommendations to do something like that? the backup server is going to be very low load too with all disk idle almost all the time except when performing a backup (for a few hours every night)

Thanks in advance for answering any of these questions

	--Ed Martin


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