Messages in this thread | | | Date | Sat, 02 Jun 2007 18:20:38 +0900 | From | Tejun Heo <> | Subject | Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md. |
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Hello,
Jens Axboe wrote: >> Would that be very different from issuing barrier and not waiting for >> its completion? For ATA and SCSI, we'll have to flush write back cache >> anyway, so I don't see how we can get performance advantage by >> implementing separate WRITE_ORDERED. I think zero-length barrier >> (haven't looked at the code yet, still recovering from jet lag :-) can >> serve as genuine barrier without the extra write tho. > > As always, it depends :-) > > If you are doing pure flush barriers, then there's no difference. Unless > you only guarantee ordering wrt previously submitted requests, in which > case you can eliminate the post flush. > > If you are doing ordered tags, then just setting the ordered bit is > enough. That is different from the barrier in that we don't need a flush > of FUA bit set.
Hmmm... I'm feeling dense. Zero-length barrier also requires only one flush to separate requests before and after it (haven't looked at the code yet, will soon). Can you enlighten me?
Thanks.
-- tejun
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