On Tue, Feb 01, 2011 at 05:02:46PM +0100, Keld Jørn Simonsen wrote: > On Tue, Feb 01, 2011 at 11:01:33AM +0100, David Brown 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. > > I don't think you get the striping performance of raid10,f2 with this > layout. And that is one of the main advantages of raid10,f2 layout. > Have you tried it out? > > As far as I can see the layout of blocks are not alternating between the > disks. You have one raid1 of sda1 and sdb2, there a file is allocated on > blocks sequentially on sda1 and then mirrored on sdb2, where it is also > sequentially allocated. That gives no striping. Well, maybe the RAID0 layer provides the adequate striping. I am noy sure, but it looks like it could hold in theory. One could try it out. One advantage of this scheme could be improved probability When 2 drives fail, eg. in the case of a 4 drive array. The probability of survival of a running system could then be enhaced form 33 % to 66 %. One problem could be the choice of always the lowest block number, which is secured in raid10,f2, but not in a raid0 over raid1 (or raid10,n2) scenario. best regards keld -- 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
