Re: [Xen-users] Xen 4.0 feature request

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Enrico Valsecchi wrote:

> > 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.
> Sorry,
> but I think that are more cleaned that if you want manage a XEN 
> consistent snapshot, XEN must have this function!
> O.K., you CAN use other external tool to have a consistent snapshots.
> Very unique mode are use LVM, BUT, if you want have use a LVM 
> snapshot, you MUST have dedicated LMV volume for ANY virtual 
> machine....
> ....good?
Following the threads in here, I think it's fairly common to have one 
or more LVM volumes defined in Dom0 for each guest. For example, if I 
had a guest called "guest", then I'd probably have LVM volumes named 
guest-root, guest-var, and so on.
The problem is, that unless you are looking from INSIDE the guest, 
then what is in the LVM volume is NOT what the guest things is in the 
volume (the only exception being when the guest has no dirty cached 
blocks in memory for that volume).
To get round this means having tools that intimately connect with 
LVM, the guest OS, and it's filing system. Given all the permutations 
possible, that just isn't likely to happen. The best I could think of 
would be a combination of tools so that from the Dom0 you could 
signal for a guest to : stop all writes, write all dirty blocks out 
to disk, signal to the host that this is done, wait for a response, 
then resume. The admin program could then signal a guest to go into a 
'clean' state (and wait for it to reach that state), make an LVM 
snapshot of it's volume, and then signal the guest to resume. Your 
LVM snapshot would then represent the guests view of the volume at 
that instant - but it still would not be clean in the way that an 
unmounted filesystem with no open files is clean.
--
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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