On 03/02/2013 06:48 PM, Stan Hoeppner wrote:

On 3/2/2013 11:07 AM, Phil Turmel wrote:

quoted

On 03/02/2013 04:15 AM, Stan Hoeppner wrote:

quoted

On 3/1/2013 10:06 AM, Adam Goryachev wrote:

...

quoted

quoted

quoted

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