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Re: [Qemu-devel] [RFC PATCH 7/8] quorum: Implement .bdrv_co_preadv/pwrit

From: Eric Blake
Subject: Re: [Qemu-devel] [RFC PATCH 7/8] quorum: Implement .bdrv_co_preadv/pwritev()
Date: Fri, 11 Nov 2016 09:08:47 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

On 11/11/2016 03:58 AM, Kevin Wolf wrote:
>> Should we add more fields to the two affected events (QUORUM_FAILURE and
>> QUORUM_REPORT_BAD)? We have to keep the existing fields for back-compat,
>> but we could add new fields that give byte-based locations for
>> management apps smart enough to use the new instead of the old
>> (particularly since the old fields are named 'sector-num' and
>> 'sectors-count').
> If there is a user for the new fields, I can do that.

Libvirt is not using quorums yet, but would definitely prefer to use
byte-based information rather than sector based (especially since the
documentation isn't specific on how much a sector is; a quorum built on
top of disks with 4k sectors may be weird to interpret if you don't know
that qemu is hard-coded to 512-byte sectors)

>>>      qapi_event_send_quorum_report_bad(type, !!msg, msg, node_name,
>>> -                                      sector_num, nb_sectors, 
>>> &error_abort);
>>> +                                      offset / BDRV_SECTOR_SIZE,
>>> +                                      bytes / BDRV_SECTOR_SIZE, 
>>> &error_abort);
>> Rounding the offset down makes sense, but rounding the bytes down can
>> lead to weird messages.  Blindly rounding it up to a sector boundary can
>> also be wrong (example: writing 2 bytes at offset 511 really affects
>> 1024 bytes when you consider that two sectors had to undergo
>> read-modify-write). Don't we have a helper routine for determining the
>> end boundary when we have to convert an offset and length to a courser
>> alignment?
> Hm, I would have to check the header files. I don't know one off the top
> of my head. If you find it, let me know.

I guess I was thinking of something like
io.c:bdrv_round_sectors_to_clusters(), but didn't readily find a
counterpart for rounding bytes to sectors or request_alignment.  Don't
know if it would help to have one, or not.

>>> @@ -462,8 +461,8 @@ static void GCC_FMT_ATTR(2, 3) quorum_err(QuorumAIOCB 
>>> *acb,
>>>      va_list ap;
>>>      va_start(ap, fmt);
>>> -    fprintf(stderr, "quorum: sector_num=%" PRId64 " nb_sectors=%d ",
>>> -            acb->sector_num, acb->nb_sectors);
>>> +    fprintf(stderr, "quorum: offset=%" PRIu64 " bytes=%" PRIu64 " ",
>>> +            acb->offset, acb->bytes);
>> Might be worth a separate patch to get rid of fprintf and use correct
>> error reporting.  But not the work for this patch.
> What would correct error reporting be in this case? A QMP event? Because
> other than that and stderr, I don't think we have any channels for error
> messages for I/O requests. We could use error_report(), but it would be
> effectively the same thing as fprintf().

Hmm, you're probably right that a QMP event would be best, if anything
is needed at all.

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

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