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Re: [Qemu-block] [PATCH v3 0/4] qcow2: async handling of fragmented io

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH v3 0/4] qcow2: async handling of fragmented io
Date: Fri, 16 Aug 2019 16:42:37 +0000

15.08.2019 18:39, Vladimir Sementsov-Ogievskiy wrote:
> 15.08.2019 17:09, Max Reitz wrote:
>> On 15.08.19 14:10, Vladimir Sementsov-Ogievskiy wrote:
>>> Hi all!
>>> Here is an asynchronous scheme for handling fragmented qcow2
>>> reads and writes. Both qcow2 read and write functions loops through
>>> sequential portions of data. The series aim it to parallelize these
>>> loops iterations.
>>> It improves performance for fragmented qcow2 images, I've tested it
>>> as described below.
>> Looks good to me, but I can’t take it yet because I need to wait for
>> Stefan’s branch to be merged, of course.
>> Speaking of which, why didn’t you add any tests for the *_part()
>> methods?  I find it a bit unsettling that nothing would have caught the
>> bug you had in v2 in patch 3.
> Hmm, any test with write to fragmented area should have caught it.. Ok,
> I'll think of something.

And now I see that it's not trivial to make such a test:

1. qcow2 write is broken when we give nonzero qiov_offset to it, but only
qcow2_write calls bdrv_co_pwritev_part, so we need to have a test where qcow2
is a file child for qcow2

2. Then, the bug leads to the beginning of the qiov will be written to all 
But our testing tool qemu-io has only "write -P" command with buffer filled with
the same one byte, so we can't catch it

Best regards,

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