Questions about RAID 6
in [Debian]
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From: Mark Allums on 26 Apr 2010 14:00 On 4/26/2010 11:57 AM, Tim Clewlow wrote: > >> I'm afraid that opinions of RAID vary widely on this list (no >> surprise) >> but you may be interested to note that we agree (a consensus) that >> software-RAID 6 is an unfortunate choice. >> > . > Is this for performance reasons or potential data loss. I can live > with slow writes, reads should not be all that affected, but data > loss is something I'd really like to avoid. > > Regards, Tim. > > Performance. RAID 6 (and 5) perform well when less than approximately 1/3 full. After that, even reads suffer. True hardware RAID can compensate somewhat, but you are contemplating mdraid. Data loss should not be an issue if your array can rebuild fast enough. RAID 6 can usually withstand the loss of two drives. I like RAID 10, but I'm considered peculiar. RAID 10 can often withstand the loss of two drives (but not always) and performs a bit better, with much more graceful degradation of performance as the volume fills up. It performs reasonably well 2/3 full. If data loss is crucial, RAIDed arrays can always lose one drive and recover, but for very important data, mirrors (RAID 1) are better. Put four drives in a RAID 1, you can suffer a loss of three drives. MAA -- To UNSUBSCRIBE, email to debian-user-REQUEST(a)lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmaster(a)lists.debian.org Archive: http://lists.debian.org/4BD5D2AA.6010109(a)allums.com
From: Mike Bird on 26 Apr 2010 14:40 On Mon April 26 2010 10:51:38 Mark Allums wrote: > RAID 6 (and 5) perform well when less than approximately 1/3 full. > After that, even reads suffer. Mark, I've been using various kinds of RAID for many many years and was not aware of that. Do you have a link to an explanation? Thanks, --Mike Bird -- To UNSUBSCRIBE, email to debian-user-REQUEST(a)lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmaster(a)lists.debian.org Archive: http://lists.debian.org/201004261137.35915.mgb-debian(a)yosemite.net
From: Stan Hoeppner on 26 Apr 2010 15:40 Mark Allums put forth on 4/26/2010 12:51 PM: > Put four drives in a RAID 1, you can suffer a loss of three drives. And you'll suffer pretty abysmal write performance as well. Also keep in mind that some software RAID implementations allow more than two drives in RAID 1, most often called a "mirror set". However, I don't know of any hardware RAID controllers that allow more than 2 drives in a RAID 1. RAID 10 yields excellent fault tolerance and a substantial boost to read and write performance. Anyone considering a 4 disk mirror set should do RAID 10 instead. -- Stan -- To UNSUBSCRIBE, email to debian-user-REQUEST(a)lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmaster(a)lists.debian.org Archive: http://lists.debian.org/4BD5E9A7.70403(a)hardwarefreak.com
From: Mike Bird on 26 Apr 2010 16:20 On Mon April 26 2010 12:29:43 Stan Hoeppner wrote: > Mark Allums put forth on 4/26/2010 12:51 PM: > > Put four drives in a RAID 1, you can suffer a loss of three drives. > > And you'll suffer pretty abysmal write performance as well. Write performance of RAID-1 is approximately as good as a simple drive, which is good enough for many applications. > Also keep in mind that some software RAID implementations allow more than > two drives in RAID 1, most often called a "mirror set". However, I don't > know of any hardware RAID controllers that allow more than 2 drives in a > RAID 1. RAID 10 yields excellent fault tolerance and a substantial boost > to read and write performance. Anyone considering a 4 disk mirror set > should do RAID 10 instead. Some of my RAIDs are N-way RAID-1 because of the superior read performance. --Mike Bird -- To UNSUBSCRIBE, email to debian-user-REQUEST(a)lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmaster(a)lists.debian.org Archive: http://lists.debian.org/201004261304.13643.mgb-debian(a)yosemite.net
From: Boyd Stephen Smith Jr. on 26 Apr 2010 17:50
On Monday 26 April 2010 09:29:28 Tim Clewlow wrote: > I'm getting ready to build a RAID 6 with 4 x 2TB drives to start, > but the intention is to add more drives as storage requirements > increase. Since you seem fine with RAID 6, I'll assume you are also fine with RAID 5. I don't know what your requirements / levels of paranoia are, but RAID 5 is probably better than RAID 6 until you are up to 6 or 7 drives; the chance of a double failure in a 5 (or less) drive array is minuscule. > I intend to use mdadm to build / run the array. Modern mdadm can migrate from RAID 5 to RAID 6 when you add the 6th/7th drive into the array. Also modem mdadm has a wealth of RAID 1/0 features that may actually be a better performance-wise than RAID 5 or RAID 6. > If an unrecoverable > read error (bad block that on disk circuitry cant resolve) is > discovered on a disk then how does mdadm handle this? It appears the > possibilities are: > 1) the disk gets marked as failed in the array - ext3 does not get > notified of a bad block This one. > I would really like to hear it is either 2 or 3 as I would prefer > not to have an entire disk immediately marked bad due to one > unrecoverable read error Sorry. > I would prefer to be notified instead so > I can still have RAID 6 protecting "most" of the data until the disk > gets replaced. You can add the failed device back into the array and it will re-sync until there is another issue with the device. Just be sure to remember which device needs replacing for when your new HW arrives. -- Boyd Stephen Smith Jr. ,= ,-_-. =. bss(a)iguanasuicide.net ((_/)o o(\_)) ICQ: 514984 YM/AIM: DaTwinkDaddy `-'(. .)`-' http://iguanasuicide.net/ \_/ |