[Gluster-users] Replication delay

[Vijay Bellur]

On 01/25/2014 02:28 PM, Pranith Kumar Karampuri wrote:
> Vijay,
>       But it seems like self-heal's fd is able to perform 'writes'. Shouldn't it be uniform if it is the problem with xfs?

The problem is not with xfs alone. It is due to a combination of several 
factors including disk sector size, xfs sector size and the nature of 
writes being performed. With cache=none, qemu does O_DIRECT open() which 
necessitates proper alignment for write operations to happen 
successfully. Self-heal does not open() with O_DIRECT and hence write 
operations initiated by self-heal go through.

-Vijay

>
> Pranith
> ----- Original Message -----
>> From: "Vijay Bellur" <vbellur at redhat.com>
>> To: "Fabio Rosati" <fabio.rosati at geminformatica.it>, "Pranith Kumar Karampuri" <pkarampu at redhat.com>
>> Cc: "Gluster-users at gluster.org List" <gluster-users at gluster.org>
>> Sent: Saturday, January 25, 2014 1:23:52 PM
>> Subject: Re: [Gluster-users] Replication delay
>>
>> On 01/24/2014 09:24 PM, Fabio Rosati wrote:
>>
>>>
>>>
>>> The block size is the same, 4096 bytes.
>>> I did some other investigation and it seems the problem happens only with
>>> VM disk images internally formatted with a blocksize of 1024 bytes. There
>>> are no problems with disk images formatted with a block size on 4096
>>> bytes. Anyway, I don't know if this is a coincidence.
>>>
>>> Do you think this could be the origin of the problem? If so, how can I
>>> solve it?
>>> In the links posted by Vijay someone suggests to start the VM with cache !=
>>> none but this will prevent live migration, AFAIK.
>>> Another solution may be to recreate the volume backing it with XFS
>>> partitions formatted with a different block size (smaller? 1024 bytes?),
>>> this would be a painful option, but if this will solve the problem, I go
>>> for it.
>>>
>>
>> A lower sector size (512) for xfs has been observed to be useful in
>> overcoming this problem.
>>
>> Another solution might be to use logical_block_size=4096 option as
>> referred here [1].
>>
>> -Vijay
>>
>> [1] https://bugzilla.redhat.com/show_bug.cgi?id=997839#c7
>>
>>
>
>