Your program only addresses large I/O (1MB) against a fairly large file
(4GB). We did that as well with Bonnie++ (2GB file 1MB I/O requests). The
results without the fsync option ran about 94 - 100 MB/s. Our concern was
creating a more real world mix of I/O. How well does the system scale
against a variety of I/O request sizes on various size files. Where we saw
the worst overall scaling was with 8K requests.
> >And please tell us some more details regarding the performance
> >I assume that you mean that the IO rate per disk slows as more
> >disks are added to an adapter? Or does the total throughput through
> >the adapter fall as more disks are added?
> No, the IO block write throughput for the system goes down as drives are
> added under this work load. We measure the system throughput not the
> per drive throughput, but one could infer the per drive throughput by
> Running bonnie++ on with 300MB files doing 8Kb sequential writes we get
> the following system wide throughput as a function of the number of
> drives attached and by number of addapters.
> One addapter
> 1 drive per addapter 127,702KB/Sec
> 2 drives per addapter 93,283 KB/Sec
> 6 drives per addapter 85,626 KB/Sec
127 megabytes/sec to a single disk? Either that's a very
fast disk, or you're using very small bytes :)
> 2 addapters
> 1 drive per addapter 92,095 KB/Sec
> 2 drives per addapter 110,956 KB/Sec
> 6 drives per addapter 106,883 KB/Sec
> 4 addapters
> 1 drive per addapter 121,125 KB/Sec
> 2 drives per addapter 117,575 KB/Sec
> 6 drives per addapter 116,570 KB/Sec
Possibly what is happening here is that a significant amount
of dirty data is being left in memory and is escaping the
measurement period. When you run the test against more disks,
the *total* amount of dirty memory is increased, so the kernel
is forced to perform more writeback within the measurement period.
So with two filesystems, you're actually performing more I/O.
You need to either ensure that all I/O is occurring *within the
measurement interval*, or make the test write so much data (wrt
main memory size) that any leftover unwritten stuff is insignificant.
bonnie++ is too complex for this work. Suggest you use
which will just write and fsync a file. Time how long that
takes. Or you could experiment with bonnie++'s fsync option.
My suggestion is to work with this workload:
for i in /mnt/1 /mnt/2 /mnt/3 /mnt/4 ...
write-and-fsync $i/foo 4000 &
which will write a 4 gig file to each disk. This will defeat
any caching effects and is just a way simpler workload, which
will allow you to test one thing in isolation.
So anyway. All this possibly explains the "negative scalability"
in the single-adapter case. For four adapters with one disk on
each, 120 megs/sec seems reasonable, assuming the sustained
write bandwidth of a single disk is 30 megs/sec.
For four adapters, six disks on each you should be doing better.
Something does appear to be wrong there.
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