Re: Raid 1 vs Raid 10 single thread performance

[NeilBrown <neilb@suse.de>]

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On Thu, 11 Sep 2014 06:48:31 +0200 Bostjan Skufca <bostjan@a2o.si> wrote:

> On 11 September 2014 02:31, NeilBrown <neilb@suse.de> wrote:
> > On Wed, 10 Sep 2014 23:24:11 +0200 Bostjan Skufca <bostjan@a2o.si> wrote:
> >> What does "properly" actually mean?
> >> I was doing some benchmarks with various raid configurations and
> >> figured out that the order of devices submitted to creation command is
> >> significant. It also makes raid10 created in such mode reliable or
> >> unreliable to a device failure (not partition failure, device failure,
> >> which means that two raid underlying devices fail at once).
> >
> > I don't think you've really explained what "properly" means.  How exactly do
> > you get better throughput?
> >
> > If you want double-speed single-thread throughput on 2 devices, then create a
> > 2-device RAID10 with "--layout=f2".
> 
> I went and retested a few things and I see I must have done something
> wrong before:
> - regardless whether I use --layout flag or not, and
> - regardless of device cli arg order at array creation time,
> = I always get double-speed single-thread throughput. Yaay!
> 
> Anyway, the thing is that regardless of -using -layout=f2 or not,
> redundancy STILL depends on the order of command line arguments passed
> to mdadm --create.
> If I do:
> - "sda1 sdb1 sda2 sdb2" - redundandcy is ok
> - "sda1 sda2 sdb1 sdb2" - redundancy fails
> 
> Is there a flag that ensures redundancy in this particular case?
> If not, don't you think the naive user (me, for example) would assume
> that code is smart enough to ensure basic redundancy, if there are at
> least two devices available?

I cannot guess what other people will assume.  I certainly cannot guard
against all possible incorrect assumptions.

If you create an array which doesn't have true redundancy you will get a
message from the kernel saying:

  %s: WARNING: %s appears to be on the same physical disk as %s.
  True protection against single-disk failure might be compromised.

Maybe mdadm could produce a similar message...


> 
> Because, if someone wants only performance and no redundancy, they
> will look no further than raid 0. But raid10 strongly hints at
> redundancy being incorporated in it. (I admit this is anecdotal, based
> on my own experience and thought flow.)

I really don't think there is any value is splitting a device into multiple
partitions and putting more than one partition per device into an array.
Have you tried using just one partition per device, making a RAID10 with
--layout=f2 ??

NeilBrown


> 
> 
> b.
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