On Wednesday 09 of May 2012 22:01:49 Daniel Pocock wrote: > There is various information about > - enterprise-class drives (either SAS or just enterprise SATA) > - the SCSI/SAS protocols themselves vs SATA > having more advanced features (e.g. for dealing with error conditions) > than the average block device > > For example, Adaptec recommends that such drives will work better with > their hardware RAID cards: > > http://ask.adaptec.com/cgi-bin/adaptec_tic.cfg/php/enduser/std_adp.php?p_f > aqid=14596 "Desktop class disk drives have an error recovery feature that > will result in a continuous retry of the drive (read or write) when an > error is encountered, such as a bad sector. In a RAID array this can > cause the RAID controller to time-out while waiting for the drive to > respond." > > and this blog: > http://www.adaptec.com/blog/?p=901 > "major advantages to enterprise drives (TLER for one) ... opt for the > enterprise drives in a RAID environment no matter what the cost of the > drive over the desktop drive" > > My question.. > > - does btrfs RAID1 actively use the more advanced features of these > drives, e.g. to work around errors without getting stuck on a bad block? There are no (short) timeouts that I know of > - if a non-RAID SAS card is used, does it matter which card is chosen? > Does btrfs work equally well with all of them? If you're using btrfs RAID, you need a HBA, not a RAID card. If the RAID card can work as a HBA (usually labelled as JBOD mode) then you're good to go. For example, HP CCISS controllers can't work in JBOD mode. If you're using the RAID feature of the card, then you need to look at general Linux support, btrfs doesn't do anything other FS don't do with the block devices. > - ignoring the better MTBF and seek times of these drives, do any of the > other features passively contribute to a better RAID experience when > using btrfs? whatever they really have high MTBF values is debatable... seek times do matter very much to btrfs, fast CPU is also a good thing to have with btrfs, especially if you want to use data compression, high node or leaf sizes > - for someone using SAS or enterprise SATA drives with Linux, I > understand btrfs gives the extra benefit of checksums, are there any > other specific benefits over using mdadm or dmraid? Because btrfs knows when the drive is misbeheaving (because of checksums) and is returning bad data, it can detect problems much faster then RAID (which doesn't use the reduncancy for checking if the data it's returning is actually correct). Both hardware and software RAID implementations depend on the drives to return IO errors. In effect, the data is safer on btrfs than regular RAID. Besides that online resize (both shrinking and extending) and (currently not implemented) ability to set redundancy level on a per file basis. In other words, with btrfs you can have a file with RAID6 redundancy and a second one with RAID10 level of redundancy in single directory. Regards, -- Hubert Kario QBS - Quality Business Software 02-656 Warszawa, ul. Ksawerów 30/85 tel. +48 (22) 646-61-51, 646-74-24 www.qbs.com.pl -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
