Re: debian software raid1

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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).

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.

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.

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.

Iordan
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