On 23/04/11 02:05, 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).
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.
Look at this to see some pictures of raid10 layouts:
<http://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10>
Raid10,far will fair worse than raid10,near when degraded. But it will
still work - your data is mirrored, and you can pull a drive without
losing anything.
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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