RAID 10 / 0+1 / 1+0 Confusion
Can someone point me towards some reasonably detailed information on RAID 0+1 and 1+0. So far I have only found one paragraph explanations. I understand that RAID 0+1 is striping followed by mirroring and RAID 1+0 (RAID 10) is mirroring followed by striping and I have seen Art Kagels preference for 1+0 on the IIUG website. Art makes the following point about 0+1 : "RAID 0+1 (ie mirroring two stripe sets to each other): If a drive fails one side of the mirror, an entire stripe set is useless and the whole stripe needs to be recovered from mirror. Also you can only recover from multiple drive failures if all failed drives are on the same side of the mirror. " I am unclear as to why one disk failing would require the recovery of the whole stripe. I have also been confused by information on some manufactures websites, specifically Compaq who seem to use RAID 0+1 and 1+0 interchangably. It seems various controllers configured to RAID 1 automatically give you 0+1 or 1+0 (depending on which document you read) if you have four or more drives. Any pointers would be appreciated Simon --------------------------------------------------------------------------- DISCLAIMER: Any Opinions expressed above are, at best, only consistent with my state of mind at the time of posting. Simon Burrows, NW England simon@calyps0.demon.co.uk http://www.calyps0.demon.co.uk ---------------------------------------------------------------------------
Simon Burrows wrote: > > Can someone point me towards some reasonably detailed information on > RAID 0+1 and 1+0. So far I have only found one paragraph explanations. > > I understand that > > RAID 0+1 is striping followed by mirroring and > RAID 1+0 (RAID 10) is mirroring followed by striping > > and I have seen Art Kagels preference for 1+0 on the IIUG website. > Art makes the following point about 0+1 : > "RAID 0+1 (ie mirroring two stripe sets to each other): > If a drive fails one side of the mirror, an entire stripe set is > useless and the whole stripe needs to be recovered from mirror. Also > you can only recover from multiple drive failures if all failed drives > are on the same side of the mirror. " > > I am unclear as to why one disk failing would require the recovery of > the whole stripe. The problem is that unless you are using hardware RAID01 the mirror software/ firmware does not know it is mirroring a stripe set and so can only recover the entire logical drive that has failed from its mirror. The logical drives that are being mirrored are the entire stripe sets so that is the unit of recovery. IFF you are using a hardware RAID01, or I suppose a sophisticated software only RAID01, then it is POSSIBLE, though not likely, for the firmware to have enough knowledge of the layout to realize that only one drive of the stripe set is down and recover only that one drive. HOWEVER, in this case you do not really have RAID01, since each drive in the stripe sets is mirror individually, what you have, functionally, is RAID10 with an unfortunate name of RAID01! There is clearly confusion about these two pseudo RAID levels, mostly because they are not officially defined levels but levels that developed in the field when users, and later vendors, combined striping and mirroring to gain the advantages of both and ended up creating these new levels. Both the mirror over stripe and stripe over mirror approaches came to be known as RAID 0+1 and it became rather confusing. About two years ago, to help clear things up, I proposed that we separate the two approaches by calling one RAID01 and the other RAID10 based on the order in which the stripe or mirror were applied, so RAID01 refers to stripe then mirror and RAID10 to mirror then stripe. Some of my messages were in response to cross-posts with RAID related newsgroups so the proposal received some wider exposure than just Informix folk. Unfortunately there is little discussion about the differences between RAID01 and RAID10 anywhere else than here though from discussions I have had with two controller manufacturers the RAID01/RAID10 as described has become a pretty standard way of referring to the stripe/mirror combinations. Art S. Kagel > I have also been confused by information on some manufactures > websites, specifically Compaq who seem to use RAID 0+1 and 1+0 > interchangably. > It seems various controllers configured to RAID 1 automatically give > you 0+1 or 1+0 (depending on which document you read) if you have four > or more drives. > > Any pointers would be appreciated > > Simon > > --------------------------------------------------------------------------- > DISCLAIMER: Any Opinions expressed above are, at best, only consistent with > my state of mind at the time of posting. > Simon Burrows, NW England simon@calyps0.demon.co.uk > http://www.calyps0.demon.co.uk > ---------------------------------------------------------------------------
Hi. I'v just finished setting up a RAID 0+1 on a old Digital Prioris.... In the manual from Digital I found a description on 0+1. RAID 0+1 writes data across the drives in the same striping fasion as RAID 0, and achieves redundancy by mirroring data similar to RAID 1. In RAID 0+1, unlike RAID 1, the data disks are also the morror disks. RAID 0+1 mirrors data back onto the data disks rotated by 1 drive. Therefore, you can use an odd number of drives in a RAID 0+1 configuration whereas you need a even number of drives in a RAID 1 configuraion. You can continue to access data in a RAID 0+1 configuration with a singel drive failure. However, you can not recover in the event of multiple drive failure. SCSI || || ______________________________________ Disk Array Controler SCSI SCSI SCSI _______________________________________ || || || Disks> Data/1 Data/2 Data/3 Mirror/3 Mirror/1 Mirror/2 Data> Block0-1 D1 Block2-3 D2 Block4-5 D3 Block4-5 M3 Block0-1 M1 Block2-3 M2 Block6-7 D1 Block8-9 D2 Block10-11 D3 Block10-11 M3 Block6-7 M1 Block8-9 M2 D1 = Data/1 M1 = Mirror/1 and so on.... I hope you find this usefull! /Marcus In article <3894afbd.9719983@news.demon.co.uk>, simon@calyps0.demon.co.uk (Simon Burrows) wrote: > > Can someone point me towards some reasonably detailed information on > RAID 0+1 and 1+0. So far I have only found one paragraph explanations. > > I understand that > > RAID 0+1 is striping followed by mirroring and > RAID 1+0 (RAID 10) is mirroring followed by striping > > and I have seen Art Kagels preference for 1+0 on the IIUG website. > Art makes the following point about 0+1 : > "RAID 0+1 (ie mirroring two stripe sets to each other): > If a drive fails one side of the mirror, an entire stripe set is > useless and the whole stripe needs to be recovered from mirror. Also > you can only recover from multiple drive failures if all failed drives > are on the same side of the mirror. " > > I am unclear as to why one disk failing would require the recovery of > the whole stripe. > > I have also been confused by information on some manufactures > websites, specifically Compaq who seem to use RAID 0+1 and 1+0 > interchangably. > It seems various controllers configured to RAID 1 automatically give > you 0+1 or 1+0 (depending on which document you read) if you have four > or more drives. > > Any pointers would be appreciated > > Simon > > ------------------------------------------------------------------------ --- > DISCLAIMER: Any Opinions expressed above are, at best, only consistent with > my state of mind at the time of posting. > Simon Burrows, NW England simon@calyps0.demon.co.uk > http://www.calyps0.demon.co.uk > ------------------------------------------------------------------------ --- > Sent via Deja.com http://www.deja.com/ Before you buy.