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Re: [Qemu-block] [Qemu-devel] [PATCH for-2.12] qcow2: Reset free_cluster
Re: [Qemu-block] [Qemu-devel] [PATCH for-2.12] qcow2: Reset free_cluster_index when allocating a new refcount block
Wed, 21 Mar 2018 14:15:57 +0100
Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)
On Wed 21 Mar 2018 02:08:23 PM CET, Eric Blake wrote:
>> - This scenario is harder to reach: in order to fill a 1-cluster
>> refcount table the size of the image needs to be larger than
>> (cluster_size³ / refcount_bits) bytes, that's 16TB with the default
>> parameters. So although it can be reproduced easily if you reduce
>> the cluster size I think it's very infrequent under normal
> Yes, 16TB for default size, but only 2M for the best-case (512-byte
> cluster, 64-bit refcount), so still easy to write a test for.
The test case is indeed very easy:
qemu-img create -f qcow2 -o cluster_size=512 hd.qcow2 16M
qemu-io -c 'write 0 8222208' hd.qcow2
qemu-io -c 'write 8222208 512' hd.qcow2
What I haven't done is debug the problem yet.
As I said I don't think this happens often in a real-world scenario, so
I wanted to fix the simple (and more common) problem first.