On Saturday July 23, mingz@xxxxxxxxxxx wrote:
> 1048576 = 1024 * 1024 = 32 * 32768. :)
^^^
>
> so it should be 32 stripe writes.
>
> ming
>
> On Fri, 2005-07-22 at 23:14 -0700, Tyler wrote:
> > By my calculations, 1048756 is *not* a multiple of 32768 (32
^^^
> > Kilobytes). Did I miscalculate?
> >
A typo somewhere :-)
> > Regards,
> > Tyler.
> >
> > Ming Zhang wrote:
> >
> > >i created a 32KB chunk size 3 disk raid5. then write this disk with a
> > >small code i wrote. i found that even i write it with 1048756 in unit,
> > >which is multiple of stripe size, it still has a lot of read when seen
> > >from iostat.
> > >sda 605.05 387.88 35143.43 384 34792
> > >sdb 611.11 323.23 35143.43 320 34792
> > >sdc 602.02 387.88 35143.43 384 34792
I wouldn't call this "a lot of read". The read requests are only 1%
of the write requests. So I would call it "some read".
There is quite a lot of complexity between the 'write' system all and
the data actually getting to the device. Presumably the Linux VM
system is flushing dirty data to the device at times other than then
end of the write request.
I think the block layer also automatically flushes devices every
200msecs.
This may be triggering flush requests which aren't stripe-aligned.
You are certainly getting the vast majority of stripes written as
whole stripes.
NeilBrown
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