Speed up reshape? (was Re: Cancel reshape?)

[John Robinson <john.robinson@anonymous.org.uk>]

On 12/01/2011 05:32, NeilBrown wrote:
> On Wed, 12 Jan 2011 04:49:35 +0000 John Robinson
> <john.robinson@anonymous.org.uk>  wrote:
[...]
>> Perhaps I can stop the reshape and restart it with the backup file
>> elsewhere. Hmm, I've an SSD I haven't installed yet... Maybe time to
>> give it a wee workout :-)
>
> That should work.
>
>   mdadm --stop ...
>   copy file
>   mdadm --assemble ...... --backup-file=/new/place

Yup. Not much of an improvement, though; the reshape is now running at 
~10MB/s instead of ~6.5MB/s, so ~27 hours to go instead of ~40 hours.

iostat tells me the original 3 discs are doing ~120 transactions per 
second, reading ~20MB/s and writing ~10MB/s, while the one new disc is 
doing ~60 tps, reading nothing (obviously) and writing ~10MB/s and the 
backup (SSD) is doing ~45 tps and writing ~20MB/s. ~120tps sounds about 
right for 120 seeks, 60 reads and 60 writes, and would explain the 
speed, but otherwise this looks odd to me - why are we reading ~60MB/s 
in total and writing ~40MB/s? I'd have expected to be reading less than 
we're writing. And why are we apparently reading and writing in blocks 
of under 512KB per device, when the backup file is 32MB? Is there 
something I can tweak to run the reshape in bigger chunks? 
sync_speed_min is already 200000...

Currently running SystemRescueCD 2.0.0, 32-bit kernel 2.6.35 and mdadm 
3.1.4. CPU usage is 2% system, 4% iowait.

Cheers,

John.