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

[xensource@xxxxxxxxxx]

Hi,

I do not fully agree :
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.

Because when you issue a backup of your VM, you will always issue a sync in the VM just before.

I had a script that for every vm listed in a file (ie /etc/xen/vm_to_backup), and issue :
1. xm sysrq <domUname> s && xm pause <domUname> (to sync the domU fs and pause it)
2. take the snapshot of the domU volume (every domU was on a dedicated lv, using linux lvm)
3. xm unpause <domUname>
4. mount the snapshot filsystem in readonly, backup the filesystem (ie using tar), unmount the snapshot and delete it.

for every backup the domU was paused for less than 1 second, and this is not "equivalent of what's on the disk at the point you kill the power" as you do not sync the fs before killing the power. Killing the power is even worst because if you do not have a battery on your raid controller, you also loose all the datas in controller cache.

This backup method is definitely not the best, and it's highly recommended to use it only journalised FS, however it's easier and far better than issuing a filesystem backup within the domU. If you have applications that manage their own cache such as DB, you cannot rely on any filesystem backup, except if you us a sync to flush the cache also in the application. With db, you should always read the recommendations from the provider : ie : use rman with oracle, put your tablespaces in backup mode with postgresql, ...


Regards,

----- Original Message -----
From: "Simon Hobson" <linux@xxxxxxxxxxxxxxxx>
To: xen-users@xxxxxxxxxxxxxxxxxxx
Sent: Saturday, 15 August, 2009 21:29:44 GMT +01:00 Amsterdam / Berlin / Bern / Rome / Stockholm / Vienna
Subject: Re: [Xen-users] snapshot backup at iscsi or dom0, which one is better?

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.

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.

If you want a usable and clean backup - do it from within the guest
(so it will include any dirty cache) and don't forget applications
like databases that typically have to be shut down, put into some
sort of backup mode, or perform a data dump in order to get a clean
backup.

--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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