Greg wrote:
Ummm... No. You're reading the results wrongly, I'm afraid :)
I would seriously doubt you'd see 1.5Gb / sec outside the datacentre.
150Mb / sec... _maybe_.
Cached reads are just that - It reads the same sector over and over.
This means it's already in drive's read cache. So what you're
seeing here is the transfer rate from the controller without
(much) SATA latency, no seek latency, no rotational latency.
That's not the test you want. You want 'buffered disk reads'.
This is the (almost real) raw performance of the drive.
Here's the results from my array, which is a 8 disk RAID5,
spread over 2 SATA controllers on a PCI-X dual Xeon (yes, I
believe overkill _is_ just a word):
hdparm -Tt /dev/sda
/dev/sda:
Timing cached reads: 2204 MB in 2.00 seconds = 1101.07 MB/sec
Timing buffered disk reads: 374 MB in 3.01 seconds = 124.31 MB/sec
The 'buffered disk reads' is the line you're looking for. And
124Mb / sec is quite good for this type of setup.
Cheers,
Allan.
Allan,
Isnt sda just one of the disks in your array??
I get:
Hdparm -Tt /dev/hda (single 40gb pata)
Timing cached reads: 2396 MB in 2.00 seconds = 1196.98 MB/sec
Timing buffered disk reads: 122 MB in 3.04 seconds = 40.14 MB/sec
Hdparm -Tt /dev/sda (160gb sata, 1st disk in 2 disk software raid)
Timing cached reads: 2412 MB in 2.00 seconds = 1204.98 MB/sec
Timing buffered disk reads: 110 MB in 3.02 seconds = 36.39 MB/sec
Hdparm -Tt /dev/md0 (60gb raid1)
Timing cached reads: 2424 MB in 2.00 seconds = 1210.97 MB/sec
Timing buffered disk reads: 110 MB in 3.02 seconds = 36.38 MB/sec
Hdparm -Tt /dev/md1 (200gb raid0)
Timing cached reads: 2408 MB in 2.00 seconds = 1201.78 MB/sec
Timing buffered disk reads: 256 MB in 3.08 seconds = 83.08 MB/sec
Greg
Greg,
I am getting on a HW SCSI Raid system:
/dev/sdd:
Timing cached reads: 2080 MB in 2.00 seconds = 1038.60 MB/sec
Timing buffered disk reads: 190 MB in 3.01 seconds = 63.03 MB/sec
On this system i get about the same performance, no matter which raid
set i test - limitation seems to be the controller.
on a SW raid5 over 4 disks (3/4 Tb)
/dev/md0:
Timing cached reads: 1440 MB in 2.00 seconds = 719.03 MB/sec
Timing buffered disk reads: 116 MB in 3.04 seconds = 38.18 MB/sec
Cheers,
Rudy
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