A little RAID experiment

[Stefan Ring]

On Thu, Jul 26, 2012, Dave Chinner <david at fromorbit.com> wrote:
>> 10001
>> 20001
>> 30001
>> 40001
>> 10002
>> 20002
>> 30002
>> 40002
>> 10003
>> 20003
>> ...
>
> That's the problem you should have reported.

I did, but then I got bashed for using RAID 5/6 and about the
specifics of hardware and everything, which shouldn't even matter, but
I let myself get dragged into this discussion.

Anyway, in the meantime I had a closer look at the actual block trace,
and it looks a bit different than the way I interpreted it at first.
It sends runs of 30-50 writes with holes in them, like so:

2, 4-5, 7, 10-12, 14, 16-17

and so on. These holes seem to be caused by the free space
fragmentation. Every once in a while -- somewhat frequently, after 30
or so blocks, as mentioned -- it switches to another allocation group.
If these blocks were contiguous, then the elevator should be able to
merge them, but the tiny holes make this impossible. So I guess
there's nothing that can be substantially improved here. The frequent
ag switches are a bit difficult for the controller to handle, but
different controllers struggle under different work loads, and there's
nothing that can be done about that. I noticed just today that the HP
SmartArray controllers handle truly random writes better than the
MegaRAID variety that I praised so much in my postings.