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Re: [Qemu-devel] Design of the blobstore

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] Design of the blobstore
Date: Thu, 15 Sep 2011 12:17:54 +0100

On Wed, Sep 14, 2011 at 6:05 PM, Stefan Berger
<address@hidden> wrote:
>  One property of the blobstore is that it has a certain required size for
> accommodating all blobs of device that want to store their blobs onto. The
> assumption is that the size of these blobs is know a-priori to the writer of
> the device code and all devices can register their space requirements with
> the blobstore during device initialization. Then gathering all the
> registered blobs' sizes plus knowing the overhead of the layout of the data
> on the disk lets QEMU calculate the total required (minimum) size that the
> image has to have to accommodate all blobs in a particular blobstore.

Libraries like tdb or gdbm come to mind.  We should be careful not to
reinvent cpio/tar or FAT :).

What about live migration?  If each VM has a LUN assigned on a SAN
then these qcow2 files add a new requirement for a shared file system.

Perhaps it makes sense to include the blobstore in the VM state data
instead?  If you take that approach then the blobstore will get
snapshotted *into* the existing qcow2 images.  Then you don't need a
shared file system for migration to work.

Can you share your design for the actual QEMU API that the TPM code
will use to manipulate the blobstore?  Is it designed to work in the
event loop while QEMU is running, or is it for rare I/O on


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