Re: Some very basic questions

[Tejun Heo <tj@kernel.org>]

Ric Wheeler wrote:
>> FS waiting for completion of all the dependent writes isn't too good
>> latency and throughput-wise tho.  It would be best if FS can indicate
>> dependencies between write commands and barrier so that barrier
>> doesn't have to empty the whole queue.  Hmm... Can someone tell me how
>> much such scheme would help?
>>
>>   
> I think that this is where SCSI ordered tags come in (or similar
> schemes). The idea would be to have tag all IO. You bump the tag, for
> example after you send down the journal data blocks to a new tag which
> is used for the commit block data sequence.
> 
> The ordering would require that lower ranked tags must all be destaged
> to persistent storage before a subsequent tag is written out.
> 
> The T13 had a microsoft proposal that is in this area:
> 
> http://www.t13.org/Documents/UploadedDocuments/docs2007/e07174r0-Write_Barrier_Command_Proposal.doc

Yeah, that's one thing although it still has the
undetected-write-errors in front of barrier problem (SCSI spec doesn't
have a way to detect that).

There's another queue, which can be considerably larger than the
on-device buffer - the block elevator queue.  Currently, as the
elevator doesn't know what's dependent on what, it has to dump the
whole content of elevator before doing barrier.  I don't know how much
it would help to do it selectively tho.

Thanks.

-- 
tejun