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SubjectRe: trying to understand READ_META, READ_SYNC, WRITE_SYNC & co
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Jens Axboe <axboe@kernel.dk> writes:

> On 2010-06-21 11:48, Christoph Hellwig wrote:
>> Now how do we use these flags in the block layer?
>>
>> - REQ_META
>>
>> The only place where we ever use this flag is inside the
>> cfq scheduler. In cfq_choose_req we use it to give a meta
>> request priority over one that doesn't have it. But before
>> that we already do the same preference check with rw_is_sync,
>> which evaluates to true for requests with that are either
>> reads or have REQ_SYNC set. So for reads the REQ_META flag
>> here effectively is a no-op, and for writes it gives less
>> priority than REQ_SYNC.
>> In addition to that we use it to account for pending metadata
>> requests in cfq_rq_enqueued/cfq_remove_request which gets
>> checked in cfq_should_preempt to give priority to a meta
>> request if the other queue doesn't have any pending meta
>> requests. But again this priority comes after a similar
>> check for sync requests that checks if the other queue has
>> been marked to have sync requests pending.
>
> It's also annotation for blktrace, so you can tell which parts of the IO
> is meta data etc. The scheduler impact is questionable, I doubt it makes
> a whole lot of difference.

Really? Even after I showed the performance impact of setting that bit
for journal I/O?

http://lkml.org/lkml/2010/4/1/344

Cheers,
Jeff


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