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Re: [Qemu-block] [Qemu-devel] [PATCH 1/1] block: fix return code for par

From: Denis V. Lunev
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 1/1] block: fix return code for partial write for Linux AIO
Date: Tue, 28 Jun 2016 17:37:21 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1

On 06/28/2016 05:22 PM, Eric Blake wrote:
On 06/28/2016 05:21 AM, Denis V. Lunev wrote:
On 06/23/2016 02:37 PM, Denis V. Lunev wrote:
Partial write most likely means that there is not space rather than
"something wrong happens". Thus it would be more natural to return
ENOSPC rather than EINVAL.

The problem actually happens with NBD server, which has reported EINVAL
rather then ENOSPC on the first error using its protocol, which makes
report to the user wrong.
I'm not stating that this patch is wrong, but I'm also wondering if NBD
code needs to be patched (the NBD protocol specifically recommends
ENOSPC rather than EINVAL on attempts to write larger than size).  In
fact, commit 29b6c3b3 may have been that fix.

this patch does not patch return code in NBD layer
directly. Here the command comes normally to QCOW2
and then below to raw-posix/linux-aio.

From the point of low level there is no error at all,
the write has partial success status. In this case
completion handler raises EINVAL and this code
is reported to the caller, i.e. to NBD and through
NBD to the client.

In terms of NBD the patch altered return code
from blk_pwrite in nbd_trip(). The same code will
be reported to all other users and thus VM will be
properly put to pause if configured on the first
failed request without reporting the error to guest.

NBD is just an example.


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