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Re: [Qemu-devel] [PATCH v2] qemu-nbd: only send a limited number of errn

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v2] qemu-nbd: only send a limited number of errno codes on the wire
Date: Fri, 08 May 2015 06:35:28 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

On 05/08/2015 04:27 AM, Paolo Bonzini wrote:
> On 08/05/2015 12:12, Markus Armbruster wrote:
>>> The corresponding patch to the NBD protocol description can be found at
>>> http://article.gmane.org/gmane.linux.drivers.nbd.general/3154.
> [...]
>>> - EFBIG is part of the universal set of errors, but it is also changed
>>>   to ENOSPC because it is pretty similar to ENOSPC or EDQUOT.
>> Perhaps debatable, but I defer to your judgement.
> EFBIG is weird anyway, and requires you (or your parents) to ignore
> SIGXFSZ.  A simpler protocol puts fewer requirements on the client.

Not only that, but if I understand correctly, the only way to trigger
SIGXFSZ or EFBIG is to compile your application to use a smaller off_t
than the maximum the system supports.  That is, it is impossible on
64-bit Linux, and on 32-bit Linux is it possible only when you use
32-bit off_t instead of 64-bit off_t.  But we are dealing with guest
disk images, and practically require 64-bit off_t to make it useful for
all but the oldest of guests.

> (In fact, we probably should ignore SIGXFSZ in QEMU and treat EFBIG like
> ENOSPC everywhere.  Should doesn't mean that it will get on anyone's
> todo list or priority list...).

and if I'm right above, it wouldn't make a difference anyway.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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