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SubjectRe: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.
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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