Re: Optimal chunk size for RAID5?

[NeilBrown <neilb@suse.de>]

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On Sun, 22 Feb 2015 08:33:02 -0600 Alireza Haghdoost <alireza@cs.umn.edu>
wrote:

> On Sun, Feb 22, 2015 at 6:30 AM, Roman Mamedov <rm@romanrm.net> wrote:
> > On Sun, 22 Feb 2015 12:31:23 +0100
> > Christer Solskogen <christer.solskogen@gmail.com> wrote:
> >
> >> There are so many different views on the internet
> >
> > ...and yet you're asking for some more? :)
> >
> >> Is there even such a thing as optimal chunk size?
> >
> > 64K should be fine:
> > http://louwrentius.com/linux-raid-level-and-chunk-size-the-benchmarks.html
> >
> 
> I have seen that people report 64K chunk size results better
> performance. However, I was not able to find why mdadm maintainers
> decided to switch into 512K default chunk size a few years ago ? Was
> that decision related to the write-intent bitmap overhead ?

No, write-intent-bitmap sizing is completely independent from chunk sizes.

I don't remember the detail for the change, but some measurement must have
gone faster with larger chunk size.

single threaded loads tend to prefer large chunk sizes.
multi-threaded small-request random IO tends to prefer smaller chunk sizes.

There is no "Optimal" without reference to a particular work load.  Or
particular hardware.

NeilBrown



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