Re: The chunk size paradox

[Wolfgang Denk <wd@denx.de>]

Dear Stan,

In message <52C5A9AA.9090300@hardwarefreak.com> you wrote:
>
> > filesystem blocks and memory pages aren't necessarily 4K, though that
> > is the most common size.
> 
> Yes, they are necessarily 4K in Linux.  Linux only supports page sized
> BIO for consistency across the memory manager and IO subsystems.  Most
> architectures which Linux currently supports have hardware page sizes
> greater than 4K, for instance IA64 supports 4k/8k/16k, even a 4GB page
> size.  But it was decided long ago to stick with 4K for a number of
> reasons, one of these is stated above.  For background on this Google is
> your friend.

Well, you can tune the page size - and if you need no file system
support (like when implementing a RAID controller card) making the
page size exactly the same as the chunk size will allow for some nice
performance optimizations (as you can avoid a lot of large memcpy()
operations).

We did this (some 6 years ago) for the (then AMCC) PPC440SPe
processors; I can't find the old document any longer on APM's web
site, but there is still a copy here ([1]) which shows the effect.

So yes, tuning the system page size can have considerable impact, but
only for special-purpose applications, and when optimising for large
sequential I/O (which appears to be how the RAID controller
manufacturers are testing / optimizing their systems).


Fact is, with a file system on top of the RAID array, and with our
typical work load of many very small files, A RAID6 with chunk size
16k will give much better results that with chunk size 64k - and
anything even bigger will be worse.


[1] ftp://ftp.denx.de/pub/demos/RAID-demo/doc/RAIDinLinux_PB_0529a.pdf

Best regards,

Wolfgang Denk

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