Re: RAID10 Performance

[Robin Hill <robin@robinhill.me.uk>]

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On Wed Mar 02, 2011 at 10:04:19AM +0100, Aaron Sowry wrote:

> Hello,
> 
> I have been testing different RAID configurations on a 2-disk setup, and
> have a couple of questions regarding performance. The information I have
> found online so far seems to contradict itself fairly regularly so I was
> hoping for a more coherent answer :)
> 
> 1) As I understand it, a RAID10 'near' configuration using two disks is
> essentially equivalent to a RAID1 configuration. Is this correct?
> 
It should be, yes, though I've not actually sat down and verified this.

> 2) Does md RAID1 support 'striped' reads? If not, is RAID1 read
> performance in any way related to the number of disks in the array?
> 
No, it doesn't support 'striped' reads (there's no real performance
advantage to doing so as you'd be losing the time skipping between
stripes that you'd gain on reading from multiple drives concurrently).
You do get a performance advantage with multiple drives in that more
requests can be handled concurrently though.

> 3) From what I have read so far, a RAID10 'far' configuration on 2 disks
> provides increased read performance over an equivalent 'near'
> configuration, however I am struggling to understand exactly why. I
> understand the difference between the 'near' and 'far' configurations,
> but not *why* this should provide any speed increases. What am I missing?
> 
The speed increase is because the read speed varies across a disk -
reading from the outer sectors is faster than reading from the inner
sectors. The 'far' configuration means that all data is available on the
outer half of the drive, so reads should mostly be served from there.

> 4) I have performed a(n admittedly fairly basic) benchmark on the same
> system under two different configurations - RAID10,n2 and RAID10,f2
> using tiobench with default settings. In short, the results showed a
> significant speed increase for single-threaded sequential reads (83Mb/s
> vs 166MB/s), some increase for single-threaded random reads (1.85Mb/s vs
> 2.25Mb/s), but a decrease for every other metric, including
> multi-threaded sequential and random reads. I was expecting write
> performance to decrease under RAID10,f2 compared to RAID10,n2, but am
> slightly confused about the multi-threaded read performance. Is it my
> expectations or my testing that needs to be reviewed?
> 
I'm not sure about that one - I'd expect multi-threaded reads to be at
least as good on a "far" layout as a "near", but I've not actually run
any benchmarks myself.

Cheers,
    Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
   / / )      | Little Jim says ....                            |
  // !!       |      "He fallen in de water !!"                 |

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