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Re: [Qemu-devel] [RFC] qemu snapshot enchancement


From: Wenchao Xia
Subject: Re: [Qemu-devel] [RFC] qemu snapshot enchancement
Date: Fri, 25 Jan 2013 17:09:44 +0800
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20130107 Thunderbird/17.0.2

于 2013-1-25 16:19, Dietmar Maurer 写道:
     Thanks for declaration, one more question:
3) will have a big image file, will you back up it or just leave it
there, not backup for internal snapshot?

Sorry, I do not understand that question. Why will I have a 'a big
image file', and what do you mean by 'backup' exactly?


To make data safe, I guess usually there would be two copy of image data,
one is the VM used, one is on another "data restore server".
There are choices about how to store the data:
One is do a full clone of image disk, at each time user want to protect his
data.
One is only do a full lone of image disk at first time, later at each time, only
copy out the delta data, which is obvious better.

Again, I would not mix "VM backup" and "VM snapshots". This is something 
different,
and it is very confusing to mix them.

But yes, in general there is 'full backup', and 'incremental backup' (delta).
But IMHO 'incremental backup' is not obviously better.

I currently also thing about storing backups to some dedup database - then you 
can
have full backups with space requirements of incremental backups.

  'incremental backup' is the right word, sorry I haven't thought
out this word to describe my idea.
  I think it is better to avoid duplicating the data at beginning.

internal snapshot increase the image file size, for eg, after internal snapshot
10G image will become 10.1G, and it seems no way to get the 100M delta
data and store it in "data restore server", so you will have do a clone of
10.1G data again. So I asked whether you have done a "snapshot data copy
to restore server" in the following.

Our current backup tool always stores the current data, so any snapshot data is
not contained in the backups.

  OK, I got it, internal snapshots are ignored.

Note: The proposed backup patches (already sent to the list) make backups 
without
creating internal/external snapshot. Consistency is guaranteed by using COW.

  I guess this can be implemented, or may already exist in 3rd party
components, such as LVM2. We may first enable qemu snapshot I/O drain
case and then let dedicated storage tools take the job.

--
Best Regards

Wenchao Xia




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