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Re: [Xen-users] Xen 4.0 feature request

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.

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....

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

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