Re: file journal fadvise

[Mark Nelson <mark.nelson@inktank.com>]

On 11/30/2014 09:26 PM, Sage Weil wrote:
> On Mon, 1 Dec 2014, ??? wrote:
>> Hi sage:
>>   For fadvise_random it only change the file readahead. I think it make
>> no sense for xfs
>> Becasue xfs don't like btrfs, the journal write always on old place(at
>> first allocated). We only can make those place contiguous.
>
> I'm thinking of the OSD journal, which can be a regular file.  I guess it
> would probably be an allocator mode, set via a XFS_XFLAG_* flag passed to
> an ioctl, which makes the delayed allocation especially unconcerned with
> keeping blocks contiguous.  It would need to be combined with the discard
> ioctl so that any journal write can be allocated wherever it is most
> convenient (hopefully contiguous to some other write).
>
> sage

Hi Sage,

Could you quick write down the steps you are thinking we'd take to 
implement this?  I'm concerned about the amount of overhead this could 
cause but I want to make sure I'm thinking about it correctly. 
Especially when trim happens and what you think/expect to happens at the 
FS and device levels.

Mark

>
>
>>
>> Thanks!
>> Jianpeng
>>
>> 2014-12-01 2:46 GMT+08:00 Sage Weil <sweil@redhat.com>:
>>> Currently, when an OSD journal is stored as a file, we preallocate it as a
>>> large contiguous extent.  That means that for every journal write we're
>>> seeking back to wherever the journal is.  That possibly not ideal for
>>> writes.  For reads it's great, but that's the last thing we care about
>>> optimizing (we only read the journal after a failure, which is very rare).
>>>
>>> I wonder if we would do better if we:
>>>
>>>   1- trim/discard the old journal contents,
>>>   2- posix_fadvise RANDOM
>>>
>>> I'm not sure what the XFS behavior is in this case, but ideally it seems
>>> what we want it to do is write the journal wherever on disk it is most
>>> convenient... ideally contiguous with some other write that it is already
>>> doing.  If fadvise random doesn't do that, perhaps there is another
>>> allocator hint we can give it that will get us that behavior...
>>>
>>> sage
>>> --
>>> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in
>>> the body of a message to majordomo@vger.kernel.org
>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> --
>> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>
>>