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RE: [Xen-users] Xen 4.0 feature request

  • To: "xen-users@xxxxxxxxxxxxxxxxxxx" <xen-users@xxxxxxxxxxxxxxxxxxx>
  • From: Nathan Eisenberg <nathan@xxxxxxxxxxxxxxxx>
  • Date: Tue, 7 Jul 2009 10:26:32 -0700
  • Accept-language: en-US
  • Acceptlanguage: en-US
  • Delivery-date: Tue, 07 Jul 2009 10:28:43 -0700
  • List-id: Xen user discussion <xen-users.lists.xensource.com>
  • Thread-index: Acn+Z4WbY+bpXaMYRYed0J3J8E2g/AAv2+ag
  • Thread-topic: [Xen-users] Xen 4.0 feature request

The fact of the matter is that without flushing the buffers and cache, the 
snapshot is essentially the same as what would result if you pulled the power 
plug (sans the messy 'memory death' results that happen in the physical world 
in the nanoseconds after RAM loses power) -and while that's hardly ideal, it's 
good enough for many purposes - databases and filesystems  are generally robust 
enough to handle uncommitted transactions.

However, there are ways to get a better snapshot.  All you need is a quick 
script that logs in to the domU,  stops the database service, runs a sync, and 
then generates the snapshot and starts the database service again.

Best Regards
Nathan Eisenberg
Sr. Systems Administrator
Atlas Networks, LLC

-----Original Message-----
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
[mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Simon Hobson
Sent: Monday, July 06, 2009 11:26 AM
To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-users] Xen 4.0 feature request

Nathan Eisenberg wrote:

>Why can't you save only one VM with LVM snapshots?  Sounds like you 
>have an odd implementation, rather that there being a problem with 
>LVM.  We export two LVs per domU - 1 'data' and 1 'swap'.  I can 
>easily snapshot the 'data' LV of any single domU without a 
>performance problem.

How do you deal with the fact that you are snapshotting a dirty state 
? Unless you are using LVM inside the DomU (in which case Xen is 
irrelevant), then when you make a snapshot, it will NOT include any 
dirty blocks in the guests cache.

Unless you collaborate with the guest, get it to stop updating the 
filesystem, and flush it's cache - then you are pretty well 
guaranteed dirty (and probably corrupt) data.

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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