On Fri, Apr 22, 2011 at 08:05:06PM -0400, Iordan Iordanov wrote: > Hi Neil, > > On 04/22/11 18:12, NeilBrown wrote: > >This is not correct. RAID10-n2 on 2 drives is exactly the same layout and > >very nearly the same speed as RAID1 on 2 drives. (I say 'very nearly' only > >because the read-balancing code is a little different and might have > >slightly > >different results). Well, I think it has some significantly different results with the different load balancing algorithms. For example the one reported in this thread. Also other bemchmarks indicate this. > >Or have you measured these two and found an actually difference? That > >would > >certainly be interesting. > > The difference that I see is probably 100% due to the different read > balancing algorithm. When I start two dd processes reading from two > separate partitions on the RAID (just so there are no buffers screwing > up my results), with RAID1, I see less than one drive worth of > sequential read speed for the two dd processes combined. > > On the other hand, with RAID10 I see the two drives being utilized > fully, and I get one drive worth of sequential read speeds for each dd > process, or a total of two drives worth of read speed for the two dd > processes. > > The numbers were something like this: > > - Single drive speed: ~130MB/s sequential read. > - Two simultaneous dd sequential reads with RAID1, bs=1024k: ~40MB/s per dd. > - Two simultaneous dd sequential reads with RAID10, bs=1024k: ~130MB/s > per dd. > > That's what I meant by better sequential reads, but perhaps I should try > to phrase it more precisely. > > >RAID10-f2 will give faster sequential reads at the cost of slower writes. The writes will not be much slower, maybe 3 % slower, and in some cases faster, according to some benchmarks. > I am not sure what RAID10-f2 on a two disk setup will look like, but I > like the idea of the drives being identical, and in the worst case, > being able to pull one drive, zero the superblock, and be left with a > drive with intact data, which only RAID10-n2 can give, if I am not mistaken. Yes, RAID10-far and RAID10-offset will not do that. However both RAID10-far and RAID10-offset will be able to run in degraded mode with just one disk, and with all data intact. raid10-far will perform similarily to raid10-near with 2 dd'sC, also a near 100 % utilization of both drives. however, with just 1 dd, raid10-far wil also give almost 100 % utilization on bothe drives, while raid10-near will give 100 % on one drive and 0 % on the other drive (I think). Also when you ar doing multiple IO, RAID10-far will tend to give you speeds for an additional sequential read above the speed of a single drive - none of the other MD raid1/10 formats would do that. > Just to follow up on our discussion on Grub v2 and booting from a RAID > device. I discovered that if I allow Grub to use UUID, occasionally, it > would try to mount a raw device for root instead of the RAID device. > Apart from the nuisance, this would probably cause mismatch_cnt to be > non-zero!! (heh heh). At any rate, the guide reflects how I deal with > that - by turning off the use of UUIDs. > > Many thanks for taking a look at the guide and sharing your thoughts! > Please let me know if you still think I should change that part where I > say that RAID10 gives me faster sequential reads, and what you would say > instead. 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
