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Re: [Xen-users] snapshot backup at iscsi or dom0, which one is better?

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Subject: Re: [Xen-users] snapshot backup at iscsi or dom0, which one is better?
From: "Fajar A. Nugraha" <fajar@xxxxxxxxx>
Date: Tue, 18 Aug 2009 09:45:10 +0700
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On Sun, Aug 16, 2009 at 2:29 AM, Simon Hobson<linux@xxxxxxxxxxxxxxxx> wrote:
> Aclhk Aclhk wrote:
>> dom0 mounts disk from iscsi server lvm. domU mounts dom0 lvm.
>> pls kindly advise which one is better.
> Neither can give you a clean backup - period.


> But beyond that, both will
> achieve the same effect - an image of a dirty disk that will need to be
> restored in it's entirety even if you only need on file from the backup.

Depending on how you use it, it might not be the same.
If you do LVM on dom0 and want to create backups on iscsi server, you
need to snapshot all PVs exported on iscsi server that belongs to the
same VG at the SAME TIME. On zfs this can be achieved using "zfs
snapshot -r". Another way to work around this is to have each dom0's
VG using only one PV.

> Either way will give you an image of a dirty disk partition, missing
> anything in the guests cache waiting to be written. And that's also ignoring
> applications (particularly databases) that have dirty data in their own
> cache.
> In short - imagine pulling the power cord. You backup will be the equivalent
> of what's on the disk at the point you kill the power.

Correct. But most some applications have some sort of journaling
method, plus disk flushes at critical times, that allows them to
cleanly recover from "pulling-the-power-plug" type of failures. Some
example of those applications : Oracle database (when using archive
log), MySQL database (when using Innodb and configured correctly),
Lotus Domino. If you're using these kind of applications then snapshot
backup is a good strategy.


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