Messages in this thread | | | Date | Sat, 20 Jan 2007 23:00:46 +0100 (CET) | From | Tim Schmielau <> | Subject | Re: Abysmal disk performance, how to debug? |
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On Sun, 21 Jan 2007, Sunil Naidu wrote:
> On 1/21/07, Tim Schmielau <tim@physik3.uni-rostock.de> 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:
Yes. You have a faster Disk that writes about 45 MB/s. But I am not sure I understand what you want to know?
> [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?
I'd guess the kernel won't make much of a difference as the time is mostly determined by RAM and disk speeds.
> Are there any other such test cases?
Well, what do you want to find out? Anyways, I am in no way expert in the field of benchmarking.
Note to Willy: I finally noticed my logic actually was not flawed. I stated why dd would report approximately doubled throughputs with buffering, while you argued why the total elapsed time would not change much.
Time to go to bed now...
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