Re: RAID performance - new kernel results - 5x SSD RAID5

[Phil Turmel <philip@turmel.org>]

On 03/02/2013 06:48 PM, Stan Hoeppner wrote:
> On 3/2/2013 11:07 AM, Phil Turmel wrote:
>> On 03/02/2013 04:15 AM, Stan Hoeppner wrote:
>>> On 3/1/2013 10:06 AM, Adam Goryachev wrote:
> ...
>>>> Is it worth reducing the chunk size from 64k down to 16k or even smaller?
>>>
>>> 64KB chunks should be fine here.  Any gains with a smaller chunk would
>>> be small, and would pale in comparison to the amount of PITA required to
>>> redo the array and everything currently sitting atop it.  Remember you'd
>>> have to destroy it and start over.  You can't change chunk size of an
>>> existing array.
>>
>> Actually, you can.  For levels 0 and 4,5,6.
> 
> First, I'll reiterate that a smaller chunk size likely is not going to
> yield real workload gains for Adam.  And it obviously would decrease his
> FIO numbers, making him think performance decreased, even if it actually
> increased slightly with his real workload.

No contest.  I have effectively no experience at these hardware
performance levels.

> Speaking strictly now from a knowledge transfer standpoint, does this
> chunk size change feature go back a ways or does it require a fairly
> recent kernel and/or mdadm?  Are there any prerequisites or special
> considerations different from any other reshape operation?

From the announcement for mdadm version 3.1, October 2009:

> It contains significant feature enhancements over 3.0.x
> 
> The brief change log is:
>    -    Support --grow to change the layout of RAID4/5/6
>    -    Support --grow to change the chunksize of raid 4/5/6
>    -    Support --grow to change level from RAID1 -> RAID5 -> RAID6 and
>         back.
>    -    Support --grow to reduce the number of devices in RAID4/5/6.
>    -    Support restart of these grow options which assembling an array 
> 	which is partially grown.
>    -    Assorted tests of this code, and of different RAID6 layouts.
> 
> Note that a 2.6.31 or later is needed to have access to these.
> Reducing devices in a RAID4/5/6 requires 2.6.32.
> Changing RAID5 to RAID1 requires 2.6.33.

So I'm sure there are plenty of older systems that can't do this, but
current distros should all have it.

Phil