ok, read/write scheduling is another task.. the best algorithm is time based (optimize to minimal time) it´s not the round robin neither the closer head algorithm if you want performace use minimal time to execute (time based) in this topic (many emails) there´s a important thing. resolve the probability problem and make it 'official', include numbers and context the best algorithm for read and write isn´t the question about probability of how many mirros can i lose, how many disks can i lose (maybe can be if we change context to more source based, MAYBE) 2011/2/1 David Brown <david@xxxxxxxxxxxxxxx>: > On 01/02/2011 14:50, Jon Nelson wrote: >> >> On Tue, Feb 1, 2011 at 4:01 AM, David Brown<david@xxxxxxxxxxxxxxx> wrote: >>> >>> On 31/01/2011 23:52, Keld Jørn Simonsen wrote: >>>> >>>> raid1+0 and Linux MD raid10 are similar, but significantly different >>>> in a number of ways. Linux MD raid10 can run on only 2 drives. >>>> Linux raid10,f2 has almost RAID0 striping performance in sequential >>>> read. >>>> You can have an odd number of drives in raid10. >>>> And you can have as many copies as you like in raid10, >>>> >>> >>> You can make raid10,f2 functionality from raid1+0 by using partitions. >>> For >>> example, to get a raid10,f2 equivalent on two drives, partition them into >>> equal halves. Then make md0 a raid1 mirror of sda1 and sdb2, and md1 a >>> raid1 mirror of sdb1 and sda2. Finally, make md2 a raid0 stripe set of >>> md0 >>> and md1. >>> >>> If you have three disks, you can do that too: >>> >>> md0 = raid1(sda1, sdb2) >>> md1 = raid1(sdb1, sdc2) >>> md2 = raid1(sdc1, sda2) >>> md3 = raid0(md0, md1, md2) >>> >>> As far as I can figure out, the performance should be pretty much the >>> same >>> (although wrapping everything in a single raid10,f2 is more convenient). >> >> The performance will not be the same because. Whenever possible, md >> reads from the outermost portion of the disk -- theoretically the >> fastest portion of the disk (by 2 or 3 times as much as the inner >> tracks) -- and in this way raid10,f2 can actually be faster than >> raid0. >> > > This would presumably apply to all raid1 arrangements, not just raid10 - > when md has a choice to read from more than one place it will prefer the > outermost place. In the arrangement I described above, the raid pairs such > as md0 each have one have on an inner partition, and one half on an outer > partition. /If/ md is smart enough, then it will do the same here and read > from the outer partition by preference. > > The question is, does md determine the "outermost" copy by track number > relative to the partition, or by absolute track number on the disk? If it > is the former, then I see your point - with my raid 1 + 0 arrangement the > innermost and outermost partitions will be viewed the same. If it is the > later, then my arrangement will work equally well. > > On a related note, if you mix an SSD and a HD (partition) in a mirror, will > md prefer to read from the SSD first? I know it is possible to use the > "write-mostly" flag to force all reads to come from the SSD (assuming it > hasn't failed), but it would be nice to get parallel reads from the HD as > well whenever the read is large enough or when there are multiple reads in > parallel. > > > -- > 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 > -- Roberto Spadim Spadim Technology / SPAEmpresarial -- 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
