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XFS: the filesystem of the future?

XFS: the filesystem of the future?

Posted Feb 3, 2012 4:34 UTC (Fri) by raven667 (subscriber, #5198)
In reply to: XFS: the filesystem of the future? by dgc
Parent article: XFS: the filesystem of the future?

> They don't need it for performance because the SCSI/SAS protocol is much more efficient than SATA and so in most cases a write cache isn't necessary.

I think you are correct on every other point but I dont think this is right. SATA is pretty much the SCSI protocol as is SAS, they are only slightly incompatible for marketing rather than technical reasons. The big performance difference historically between consumer (IDE) and enterprise (SCSI) drives was tagged command queuing which is now very common in SATA drives as well although it wasn't so common in early SATA implementations. A tagged command queue allows the drive to implement an elevator which is a big win against a naive implementation without one.


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XFS: the filesystem of the future?

Posted Feb 3, 2012 12:35 UTC (Fri) by Jonno (subscriber, #49613) [Link]

Actually, SATA (Serial ATA) uses a slightly extended ATA command set over a serial bus, while SAS (Serial Attached Scsi) uses the SCSI command set over the same serial bus.

The SCSI command set is generally considered "better" than the ATA command set, though the difference isn't quite as large as the grand parent suggests. Write caches are still beneficial for SCSI (including SAS) performance, but the difference is not quite as large with SCSI as with ATA. That, as well as the fact that the average enterprise customer are more concerned about reliability than the average home user, are the reason that most SAS drives have write cache disabled by default, while most SATA drives have write cache enabled by default.

XFS: the filesystem of the future?

Posted Feb 3, 2012 17:30 UTC (Fri) by raven667 (subscriber, #5198) [Link]

The "extended ATA" command set that's used on SATA devices not operating in Legacy IDE mode is the SCSI command set. This goes all the way back to ATAPI which is the SCSI command set encapsulated with the IDE bus protocol. Drives and controllers are capable of speaking either SATA-II or SAS protocols without any cost difference AFAICT but don't, for largely marketing reasons rather than engineering ones. As I was saying before, having a command queue on the drive allows for the drive to have an IO elevator which is _the_ big performance win, details about how commands are named and whatnot is not really an important factor.


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