Re: debian software raid1

["Keld Jørn Simonsen" <keld@keldix.com>]

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