[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Qemu-block] [Qemu-devel] [PATCH v10 09/17] qcow2: Optimize write ze

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v10 09/17] qcow2: Optimize write zero of unaligned tail cluster
Date: Wed, 3 May 2017 21:47:16 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.0

On 04/28/2017 04:24 PM, Eric Blake wrote:

>>> +echo
>>> +echo == unaligned image tail cluster, no allocation needed ==
>>> +
>>> +CLUSTER_SIZE=1024 TEST_IMG="$TEST_IMG.base" _make_test_img $((size + 1024))
>> Any reason for the CLUSTER_SIZE? It passes with 64 kB as well, and I
>> actually think that would be a better test because as it is, the image's
>> "unaligned" tail is exactly one cluster (so it isn't really unaligned).
> Uhhh - copy-and-paste?  You're right, that 1024 is too small for what
> I'm actually doing with it.  :(

Actually, the whole test defaults to 4k clusters except when overridden,
but I did learn while hammering at things that we don't have a nice way
to tell that a backing file slightly shorter than the active file
behaves as though we read all zeros for the difference. I'm thinking of
proposing an RFC patch introducing BDRV_BLOCK_EOF, which gets set any
time bdrv_get_block_status() clamps the returns *pnum because it hit
EOF, to see what optimizations fall out elsewhere in the tree as a
result.  (It may not be worth it in the end, but we'll see).

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

Attachment: signature.asc
Description: OpenPGP digital signature

reply via email to

[Prev in Thread] Current Thread [Next in Thread]