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To: "Brad Voth" <brad@voth.name>
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From: Bryan Henderson <hbryan@us.ibm.com>
Date: Fri, 19 Dec 2008 11:03:56 -0800
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"Brad Voth" <brad@voth.name> wrote on 12/18/2008 01:36:12 PM:

> I can see the desire to have the snapshot at the filesystem level to
> ensure that the filesystem knows it is consistent.  However, this 
> can be a duplication of effort because of the need to have snapshots
> at the block device level for non-fs devices.  Such as raw logical 
> devices for say a database.  I would think that a viable consistent 
> solution would be to have the block device snapshot mechanism have a
> hook into the filesystem api to say, "I'm preparing to take a 
> snapshot, please quiesce and return" <take block snapshot> "You may 
> now resume, Mr. Filesystem"

But that's a layering violation.  First of all, a block device doesn't 
know what a filesystem is, so you'd have to generalize it to making these 
requests to whatever has the block device in some sense open.  I don't see 
that this sequence gains you anything over the sequence I described where 
the user makes the request of the filesystem driver and the filesystem 
driver, while it has the filesystem quiesced, issues an order to the block 
device to make a snapshot.

>In this way there is a single interface for users to take snapshots
>of the block device level whether it is a raw logical volume, or a
>filesystem sitting on a logical volume.

I don't think it makes sense to have a common means of snapshotting any of 
the various things that might reside on a block device; for some, I can't 
even think of a meaningful concept of snapshotting the information on that 
device.  E.g. where it's one of 4 disks on which some filesystem resides.

--
Bryan Henderson                     IBM Almaden Research Center
San Jose CA                         Storage Systems

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