Mark Woodward wrote:
There are a couple projects that do this. They are pretty much based
on ddsnap. You can google it.
In LVM2 world, it is fairly trivial to do what you want to do.
---
I figured it was likely -- I as LVM2 has to to know what blocks
change to make realtime snapshots. I just am trying to figure out how
to get a list of those blocks -- can I query some util and get the blocks
that are different at that point? I was figuring on using that with
a blockmap of the fs, to get files that have changed, as I'm wanting to
export
the files for smb(win client ) usage.
(1) create a virtual disk.
(2) take the "old" snapshot.
(3) write to lvdisk
(4) take the "new" snapshot.
At this stage the COW device of the "old" snapshot has all the data
that has changed up to and including the "new" snapshot. You can back
that up. As a differential. Then delete the "old" snapshot. The "new"
snapshot is now renamed to the old snapshot.
----
Now here's a confusion -- back it up as a differential? Do you
mean from a backup utility or going from some list of blocks that have
changed?
Take the next "new" snapshot. The renamed "old" snapshot has the
changes since the previous snapshot up to and including the latest
"new" snapshot. Just repeat this process, and you can do incremental
backups of your LVM disks.
----
I'm sorta already doing the above -- it's just that I'm doing my 'diff'
with 'rsync' and it's dog-slow. 100-120 minutes for ~800GB resulting in
about 2.5G of diff. Then I shuffle that off to another static vol sized for
the content -- and the 'cp' usually takes about 60-70 seconds.
What's hurting me is that "incremental backup" by having to scan the
file
system.
The biggest issue with performance is the COW aspect of snapshots. I
have found using 64K chunk sizes greatly increase performance by
reducing COW to snapshots. The default size if 4K.
----
I didn't know it was that low as a default -- but am using 64K
already --
as that's my RAID's 'chunksize' (I thought about experimenting with
larger sizes, but would like it to run in a reasonable time first.
Also I a relevant question 0-- when I do a dmsetup list, I see a
bunch of
cow volumes for drives that I **had** snaps going from at one point.
Seems like
the COW volumes didn't go away when halted...though it looks like, from
the dates, that maybe they get cleaned up at a boot(?)
I only have 1 snapshot going but I see 14 cow partitions....looking like
VG-Home (254, 3)
VG-Home--2012.09.30--00.52.54 (254, 50)
VG-Home--2012.09.30--00.52.54-cow (254, 51)
VG-Home--2012.10.01--04.58.11 (254, 52)
VG-Home--2012.10.01--04.58.11-cow (254, 53)
VG-Home--2012.10.02--07.22.14 (254, 54)
VG-Home--2012.10.02--07.22.14-cow (254, 55)
VG-Home--2012.10.03--09.08.27 (254, 56)
VG-Home--2012.10.03--09.08.27-cow (254, 57)
VG-Home-real (254, 2)
So would those be the list of blocks that changed upto the point they
were halted?
Do I need to worry about those "cow" vols taking up space?
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