[Top][All Lists]
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH] qed: do not evict in-use L2 table cache entries
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH] qed: do not evict in-use L2 table cache entries |
Date: |
Thu, 01 Mar 2012 17:10:57 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:10.0.1) Gecko/20120209 Thunderbird/10.0.1 |
Am 27.02.2012 14:16, schrieb Stefan Hajnoczi:
> The L2 table cache reduces QED metadata reads that would be required
> when translating LBAs to offsets into the image file. Since requests
> execute in parallel it is possible to share an L2 table between multiple
> requests.
>
> There is a potential data corruption issue when an in-use L2 table is
> evicted from the cache because the following situation occurs:
>
> 1. An allocating write performs an update to L2 table "A".
>
> 2. Another request needs L2 table "B" and causes table "A" to be
> evicted.
>
> 3. A new read request needs L2 table "A" but it is not cached.
>
> As a result the L2 update from #1 can overlap with the L2 fetch from #3.
> We must avoid doing overlapping I/O requests here since the worst case
> outcome is that the L2 fetch completes before the L2 update and yields
> stale data. In that case we would effectively discard the L2 update and
> lose data clusters!
>
> Thanks to Benoît Canet <address@hidden> for extensive testing
> and debugging which lead to discovery of this bug.
>
> Reported-by: Benoît Canet <address@hidden>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
Thanks, applied to the block branch.
How about a qemu-iotests case?
Kevin