Neil Brown wrote:
> On Wednesday December 6, jamesb@xxxxxxxxxxxx wrote:
>> Is there any reason the chunk size has to be a power of two?
>
> It makes division a lot easier and faster.
Thanks for the reply.
>> We're trying to use md devices to stripe over hardware RAID5 arrays, so
>> to align the I/O correctly, I want to set the chunk size to (n - 1) *
>> stripe_size for the underlying array. Which is impossible to do because
>> mdadm (and the kernel) insist on the chunk size being a power of
>> two.
>
> Surely you want to set chunk_size to 1 * stripe_size for the
> underlying array ... or maybe 1/(n-1) * stripe_size. But certainly
> not (n-1) * stripe_size. That wouldn't make any sense at all.
I suspect I haven't explained myself very clearly... I'm trying to
stripe over multiple separate hardware raid5 luns using md... see below
for details.
>> I haven't tried just removing the kernel checks yet ... is there a
>> reason why md is limited to power of two chunk sizes?
>
> You would need to check all the div and mod operations based on chunk
> size and make sure they don't implicitly assume it is a power of 2.
OK, thanks.
> What exactly is the geometry of the array you wan to access with md??
four 7+1 raid5 luns (from multiple controllers etc) with a stripe size
of 128K, and I am then striping across all four luns with md.
So I want to set the md chunk_size to 896K so we write a full stripe (7
* 128K) to each lun before moving on to the next.
Testing on other volume managers (IRIX/Linux with xvm) has shown
aligning the I/O like this improves performance for our workload (lots
of large files with streaming reads and writes), but I can't use xvm for
this particular system.
Thanks, James
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