Re: debian software raid1

[David Brown <david.brown@hesbynett.no>]

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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