On Sun, 22 Feb 2015 08:33:02 -0600 Alireza Haghdoost <alireza@xxxxxxxxxx> wrote: > On Sun, Feb 22, 2015 at 6:30 AM, Roman Mamedov <rm@xxxxxxxxxxx> wrote: > > On Sun, 22 Feb 2015 12:31:23 +0100 > > Christer Solskogen <christer.solskogen@xxxxxxxxx> 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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