} -----Original Message----- } From: linux-raid-owner@xxxxxxxxxxxxxxx [mailto:linux-raid- } owner@xxxxxxxxxxxxxxx] On Behalf Of Jens Axboe } Sent: Saturday, June 02, 2007 10:35 AM } To: Tejun Heo } Cc: David Chinner; david@xxxxxxx; Phillip Susi; Neil Brown; linux- } fsdevel@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; dm- } devel@xxxxxxxxxx; linux-raid@xxxxxxxxxxxxxxx; Stefan Bader; Andreas Dilger } Subject: Re: [RFD] BIO_RW_BARRIER - what it means for devices, } filesystems, and dm/md. } } On Sat, Jun 02 2007, Tejun Heo wrote: } > 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? } } Yeah, that's what the zero-length barrier implementation I posted does. } Not sure if you have a question beyond that, if so fire away :-) } } -- } Jens Axboe I must admit I have only read some of the barrier related posts, so this issue may have been covered. If so, sorry. What I have read seems to be related to a single disk. What if a logical disk is used (md, LVM, ...)? If a barrier is issued to a logical disk and that driver issues barriers to all related devices (logical or physical), all the devices MUST honor the barrier together. If 1 device crosses the barrier before another reaches the barrier, corruption should be assumed. It seems to me each block device that represents more than 2 other devices must do a flush at a barrier so that all devices will cross the barrier at the same time. Guy - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
