I can't speak to all of these, but... On Wed, May 9, 2012 at 4:00 PM, Daniel Pocock <daniel@xxxxxxxxxxxxx> 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_faqid=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 Linux md RAID actively use the more advanced features of these > drives, e.g. to work around errors? TLER and its ilk simply give up quickly on errors. This may be good for a RAID card that otherwise would reset itself if it doesn't get a timely response from a drive, but it can be bad for md RAID. It essentially increases the chance that you won't be able to rebuild, you lose drive A of a 2 x 3TB RAID 1, and then during rebuild drive B has an error and the disk gives up after 7 seconds, rather than doing all of its fancy off-sector reads and whatever else it would normally do to save your last good copy. > > - if a non-RAID SAS card is used, does it matter which card is chosen? > Does md work equally well with all of them? Yes, I believe md raid would work equally well on all SAS HBAs, however the cards themselves vary in performance. Some cards that have simple RAID built-in can be flashed to a dumb card in order to reclaim more card memory (LSI "IR mode" cards), but the performance gain is generally minimal > > - 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 md? Not that I know of, but I'd be interested in hearing what others think. > > - for someone using SAS or enterprise SATA drives with Linux, is there > any particular benefit to using md RAID, dmraid or filesystem (e.g. > btrfs) RAID (apart from the btrfs having checksums)? As opposed to hardware RAID? The main thing I think of is freedom from vendor lock-in. If you lose your card you don't have to run around finding another that is compatible with the hardware RAID's on-disk metadata format that was deprecated last year. Last I checked, performance was pretty great with md, and you can get fancy and spread your array across multiple controllers and things like that. Finally, md RAID tends to have a better feature set than the hardware, for example N-disk mirrors. I like running a 3 way mirror over 2 way + hotspare. > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
