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Re: [Qemu-devel] [PATCH v2 7/9] blkdebug: Add pass-through write_zero an

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH v2 7/9] blkdebug: Add pass-through write_zero and discard support
Date: Fri, 18 Nov 2016 00:27:47 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

On 17.11.2016 21:14, Eric Blake wrote:
> In order to test the effects of artificial geometry constraints
> on operations like write zero or discard, we first need blkdebug
> to manage these actions.  Ideally, it would be nice to let these
> operations also react to injected errors like read/write/flush,
> but it is not trivial to turn bdrv_aio error injection (where
> we return BlockAIOCB*) into bdrv_co (where we return int), not
> to mention the fact that I don't want to conflict with Kevin's
> concurrent work on refactoring away from bdrv_aio.  So for now,
> the operations merely have a TODO comment for adding error
> injection.
> However, one thing we CAN test is the contract promised by the
> block layer; namely, if a device has specified limits on
> alignment or maximum size, then those limits must be obeyed (for
> now, the blkdebug driver merely inherits limits from whatever it
> is wrapping, but the next patch will further enhance it to allow
> specific limit overrides).
> Tested by setting up an NBD server with export 'foo', then invoking:
> $ ./qemu-io
> qemu-io> open -o driver=blkdebug blkdebug::nbd://localhost:10809/foo
> qemu-io> d 0 15M
> qemu-io> w -z 0 15M
> Pre-patch, the server never sees the discard (it was silently
> eaten by the block layer); post-patch it is passed across the
> wire.  Likewise, pre-patch the write is always passed with
> NBD_WRITE (with 15M of zeroes on the wire), while post-patch
> it can utilize NBD_WRITE_ZEROES (for less traffic).
> Signed-off-by: Eric Blake <address@hidden>
> ---
>  block/blkdebug.c | 61 
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 61 insertions(+)

I probably have to take my R-b back, since this patch breaks iotest 98.


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