Re: Abysmal disk performance, how to debug?

[Sunil Naidu]

On 1/21/07, Tim Schmielau <tim@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> Note that these dd "benchmarks" are completely bogus, because the data
> doesn't actually get written to disk in that time. For some enlightening
> data, try
>
>   time dd if=/dev/zero of=/tmp/1GB bs=1M count=1024; time sync
>
> The dd returns as soon as all data could be buffered in RAM. Only sync
> will show how long it takes to actually write out the data to disk.
> also explains why you see better results is writeout starts earlier.

I am still getting better I feel:

[sukhoi@Typhoon ~]$ time dd if=/dev/zero of=/tmp/1GB bs=1M count=1024; time sync
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 19.5007 seconds, 55.1 MB/s

real    0m20.439s
user    0m0.004s
sys     0m4.535s

real    0m4.625s
user    0m0.000s
sys     0m0.125s


[sukhoi@Typhoon ~]$ time dd if=/dev/zero of=/tmp/1GB bs=1M count=1024 | sync
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 20.8707 seconds, 51.4 MB/s

real    0m22.449s
user    0m0.002s
sys     0m4.922s


Linux used here is not 2.6.20-rc5, but it's a FC6 2.6.19 binary. Shall
post the results with 2.6.20-rc5.

BTW, does the results vary with a customized kernel (configured w.r.t
Processor & Hardware) than a generic kernel like FC6?

Are there any other such test cases?

> Tim

Thanks,

~Akula2
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