the problem for benchmark on linux (mdadm) raid implementation (raid1 vs raid0) is: raid1 read balance(closest head) is diferent from raid0 read balance(stripe) algorithm it´s good but can´t be as fast as raid0 for read intensive (sequential) the problem of closest head algorithm is it can´t paralelize reads like raid0 paralelize 2011/2/1 Roberto Spadim <roberto@xxxxxxxxxxxxx>: >> 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 > -- 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
