Messages in this thread | | | From | (bill davidsen) | Subject | Re: libata in 2.4.24? | Date | 2 Dec 2003 22:34:20 GMT |
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In article <877k1f9e1g.fsf@stark.dyndns.tv>, Greg Stark <gsstark@mit.edu> wrote: | Jeff Garzik <jgarzik@pobox.com> writes:
| > > This doesn't happen with SCSI disks where multiple requests can be pending so | > > there's no urgency to reporting a false success. The request doesn't complete | > > until the write hits disk. As a result SCSI disks are reliable for database | > > operation and IDE disks aren't unless write caching is disabled. | > | > This is not really true. | > | > Regardless of TCQ, if the OS driver has not issued a FLUSH CACHE (IDE) | > or SYNCHRONIZE CACHE (SCSI), then the data is not guaranteed to be on | > the disk media. Plain and simple. | | That doesn't agree with people's experience. People seem to find that SCSI | drives never cache writes. This sort of makes sense since there's just not | much reason to report a write success before the write can be performed. | There's no performance advantage as long as more requests can be queued up.
I hope you mean the drives don't report completion until the data is on the platter, clearly the data is cached in the drive until it can be written. | | | > If fsync(2) returns without a flush-cache, then your data is not | > guaranteed to be on the disk. And as you noted, flush-cache destroys | > performance. | | It's my understanding that it doesn't. There was some discussion in the past | month about making the drivers issue syncs for journalled filesystems, but | even then the idea of adding it to fsync or O_SYNC files wasn't the | motivation.
With O_SYNC files there is the possibility of having a don't cache bit in the packet to the drive, even with write caching. With fsync I don't see any way to do it after the fact for only some of the data in the drive cache. That's just an observation.
Clearly with a completion status coming back after actual completion O_SYNC or fsync reduce to "wait for the ack from the drive." | | | > There are three levels: | > | > a) Data is successfully transferred to the controller/drive queue (TCQ). | > b) Data is successfully transferred to the drive's internal buffers. | > c) The drive successfully transfers data to the media. | | Only the third is of interest to Postgres or other databases. In fact, I | suspect only the third is of interest to other systems that are supposed to be | reliable like MTAs etc. I think Wietse and others would be shocked if they | were told fsync wasn't guaranteed to have waited until the writes had actually | hit the media.
I think for reliability fsync has to flush cache, regardless of the performance hit. I think a drive would be unusably slow if you did it after each O_SYNC write, so that's probably not practical. Clearly the best solution is a full SCSI implementation over PATA/SATA, but that would eliminate some of the justification for SCSI devices at premium prices. -- bill davidsen <davidsen@tmr.com> CTO, TMR Associates, Inc Doing interesting things with little computers since 1979. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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