On Tue, 18 Dec 2007, Jon Nelson wrote:
On 12/18/07, Thiemo Nagel <thiemo.nagel@xxxxxxxxx> wrote:
Performance of the raw device is fair:
# dd if=/dev/md2 of=/dev/zero bs=128k count=64k
8589934592 bytes (8.6 GB) copied, 15.6071 seconds, 550 MB/s
Somewhat less through ext3 (created with -E stride=64):
# dd if=largetestfile of=/dev/zero bs=128k count=64k
8589934592 bytes (8.6 GB) copied, 26.4103 seconds, 325 MB/s
Quite slow?
10 disks (raptors) raid 5 on regular sata controllers:
# dd if=/dev/md3 of=/dev/zero bs=128k count=64k
8589934592 bytes (8.6 GB) copied, 10.718 seconds, 801 MB/s
# dd if=bigfile of=/dev/zero bs=128k count=64k
3640379392 bytes (3.6 GB) copied, 6.58454 seconds, 553 MB/s
Interesting. Any ideas what could be the reason? How much do you get
from a single drive? -- The Samsung HD501LJ that I'm using gives
~84MB/s when reading from the beginning of the disk.
With RAID 5 I'm getting slightly better results (though I really wonder
why, since naively I would expect identical read performance) but that
does only account for a small part of the difference:
16k read 64k write
chunk
size RAID 5 RAID 6 RAID 5 RAID 6
128k 492 497 268 270
256k 615 530 288 270
512k 625 607 230 174
1024k 650 620 170 75
It strikes me that these numbers are meaningless without knowing if
that is actual data-to-disk or data-to-memcache-and-some-to-disk-too.
Later versions of 'dd' offer 'conv=fdatasync' which is really handy
(call fdatasync on the output file, syncing JUST the one file, right
before close). Otherwise, oflags=direct will (try) to bypass the
page/block cache.
I can get really impressive numbers, too (over 200MB/s on a single
disk capable of 70MB/s) when I (mis)use dd without fdatasync, et al.
The variation in reported performance can be really huge without
understanding that you aren't actually testing the DISK I/O but *some*
disk I/O and *some* memory caching.
Ok-- How's this for caching, a DD over the entire RAID device:
$ /usr/bin/time dd if=/dev/zero of=file bs=1M
dd: writing `file': No space left on device
1070704+0 records in
1070703+0 records out
1122713473024 bytes (1.1 TB) copied, 2565.89 seconds, 438 MB/s
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