Messages in this thread | | | Date | Thu, 03 Feb 2005 17:40:23 +0000 | From | Nuno Silva <> | Subject | Re: Drive performance bottleneck |
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Ian Godin wrote: > > I am trying to get very fast disk drive performance and I am seeing > some interesting bottlenecks. We are trying to get 800 MB/sec or more > (yes, that is megabytes per second). We are currently using PCI-Express > with a 16 drive raid card (SATA drives). We have achieved that speed, > but only through the SG (SCSI generic) driver. This is running the > stock 2.6.10 kernel. And the device is not mounted as a file system. I > also set the read ahead size on the device to 16KB (which speeds things > up a lot):
I was trying to reproduce but got distracted by this: (use page down, if you just want to see the odd result)
puma:/tmp/dd# sg_map /dev/sg0 /dev/sda /dev/sg1 /dev/sdb /dev/sg2 /dev/scd0 /dev/sg3 /dev/sdc puma:/tmp/dd# time sg_dd if=/dev/sg1 of=/tmp/dd/sg1 bs=64k count=1000 Reducing read to 64 blocks per loop 1000+0 records in 1000+0 records out
real 0m0.187s user 0m0.001s sys 0m0.141s puma:/tmp/dd# time dd if=/dev/sdb of=/tmp/dd/sdb bs=64k count=1000 1000+0 records in 1000+0 records out 65536000 bytes transferred in 1.203468 seconds (54455956 bytes/sec)
real 0m1.219s user 0m0.001s sys 0m0.138s puma:/tmp/dd# ls -l total 128000 -rw-r--r-- 1 root root 65536000 Feb 3 17:16 sdb -rw-r--r-- 1 root root 65536000 Feb 3 17:16 sg1 puma:/tmp/dd# md5sum * ec31224970ddd3fb74501c8e68327e7b sdb 60d4689227d60e6122f1ffe0ec1b2ad7 sg1 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
See? dd from sdb is not the same as sg1! Is this supposed to happen?
About the 900MB/sec: This same sg1 (= sdb, which is a single hitachi sata hdd) performes like this:
puma:/tmp/dd# time sg_dd if=/dev/sg1 of=/dev/null bs=64k count=1000000 time=1 Reducing read to 64 blocks per loop time to transfer data was 69.784784 secs, 939.12 MB/sec 1000000+0 records in 1000000+0 records out
real 1m9.787s user 0m0.063s sys 0m58.115s
I can assure you that this drive can't do more than 60MB/sec sustained.
My only conclusion is that sg (or sg_dd) is broken? ;)
Peace, Nuno Silva - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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