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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