Re: trying to understand READ_META, READ_SYNC, WRITE_SYNC & co

[Jens Axboe]

On 21/06/10 21.14, Christoph Hellwig wrote:
> On Mon, Jun 21, 2010 at 08:56:55PM +0200, Jens Axboe wrote:
>> FWIW, Windows marks meta data writes and they go out with FUA set
>> on SATA disks. And SATA firmware prioritizes FUA writes, it's essentially
>> a priority bit as well as a platter access bit. So at least we have some
>> one else using a meta data boost. I agree that it would be a lot more
>> trivial to add the annotations if they didn't have scheduler impact
>> as well, but I still think it's a sane thing.
> 
> And we still disable the FUA bit in libata unless people set a
> non-standard module option..

Yes, but that's a separate thing. The point is that boosting meta data
IO is done by others as well. That we don't fully do the same on the
hw side is a different story. That doesn't mean that the io scheduler
boost isn't useful.

>>>> Reads are sync by nature in the block layer, so they don't get that
>>>> special annotation.
>>>
>>> Well, we do give them this special annotation in a few places, but we
>>> don't actually use it.
>>
>> For unplugging?
> 
> We use the explicit unplugging, yes - but READ_META also includes
> REQ_SYNC which is not used anywhere.

That does look superfluous.

>>> But that leaves the question why disabling the idling logical for
>>> data integrity ->writepage is fine?  This gets called from ->fsync
>>> or O_SYNC writes and will have the same impact as O_DIRECT writes.
>>
>> We have never enabled idling for those. O_SYNC should get a nice
>> boost too, it just needs to be benchmarked and tested and then
>> there would be no reason not to add it.
> 
> We've only started using any kind of sync tag last year in ->writepage in
> commit a64c8610bd3b753c6aff58f51c04cdf0ae478c18 "block_write_full_page:
> Use synchronous writes for WBC_SYNC_ALL writebacks", switching from
> WRITE_SYNC to WRITE_SYNC_PLUG a bit later in commit
> 6e34eeddf7deec1444bbddab533f03f520d8458c "block_write_full_page: switch
> synchronous writes to use WRITE_SYNC_PLUG"
> 
> Before that we used plain WRITE, which had the normal idling logic.

Plain write does not idle.

-- 
Jens Axboe

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