> For sequential reading, this is not true. For random reading and > writing I agree with you in theory, but benchmarks show that it is not > so, at least for Linux RAID, viz the above URL. i agree with you, since linux algorith for raid1 is closest head, not round robin or time based there´s some patch on internet (google it: round robin raid1 linux) for roundrobin, but none for time based =( it´s a point of optimization of today raid1 algorithm round robin (may be at this mail list) http://www.spinics.net/lists/raid/msg30003.html 2011/2/1 Keld Jørn Simonsen <keld@xxxxxxxxxx>: > 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 > -- 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
