On Fri, Apr 22, 2011 at 08:05:06PM -0400, Iordan Iordanov wrote:

Hi Neil,

On 04/22/11 18:12, NeilBrown wrote:

quoted

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.

quoted

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.

quoted

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