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Re: [linux-lvm] access or interface to list of blocks that have, changed via C.O.W.?



On 10/03/2012 10:52 PM, Linda Walsh wrote:
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.
Well, I can honestly say that you are doing it the hard way. If you are connecting to a Linux box through samba, you can log the file changes.


(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?
I was talking about backing up the raw block level device.



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 file system is the hard way.

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?

If they are active, not only are they taking up space, but they are also being updated with every write.




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