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From: Christian on 20 Oct 2006 12:50
Hello lkml!

I've just tested the disk throughput on my machine using hdparm and found some
puzzling results. I have a two disk nvidia fakeraid 0 configuration using
dmraid and lvm.

When testing the single disks, I get good results just like when testing the
dmraid mapping. When I test the throughput on a raid partition or the LVM
Volumes I get very bad performance. Has anybody seen something like this
before?


single disks: (good results)

user(a)ubuntu:~$ sudo hdparm -Tt /dev/sd[ab]

/dev/sda:
Timing cached reads: 3384 MB in 2.00 seconds = 1693.17 MB/sec
Timing buffered disk reads: 210 MB in 3.02 seconds = 69.56 MB/sec

/dev/sdb:
Timing cached reads: 4096 MB in 2.00 seconds = 2049.10 MB/sec
Timing buffered disk reads: 208 MB in 3.01 seconds = 69.01 MB/sec


dmraid mapping: (good results)

user(a)ubuntu:~$ sudo hdparm -Tt /dev/mapper/nvidia_cbcaiacc

/dev/mapper/nvidia_cbcaiacc:
Timing cached reads: 3380 MB in 2.00 seconds = 1690.50 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate ioctl for
device
Timing buffered disk reads: 392 MB in 3.01 seconds = 130.05 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate ioctl for
device


third (or any other) partition on raid (bad throughput):

user(a)ubuntu:~$ sudo hdparm -Tt /dev/mapper/nvidia_cbcaiacc3

/dev/mapper/nvidia_cbcaiacc3:
Timing cached reads: 3272 MB in 2.00 seconds = 1636.72 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate ioctl for
device
Timing buffered disk reads: 158 MB in 3.00 seconds = 52.63 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate ioctl for
device


LVM Volume (even worse)

user(a)ubuntu:~$ sudo hdparm -Tt /dev/VolGroup00/LogVol00

/dev/VolGroup00/LogVol00:
Timing cached reads: 3636 MB in 2.00 seconds = 1818.96 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate ioctl for
device
Timing buffered disk reads: 114 MB in 3.01 seconds = 37.82 MB/sec
HDIO_DRIVE_CMD(null) (wait for flush complete) failed: Inappropriate ioctl for
device

So on LogVol00 residing on a two disk striped set I get only 38MB/s
throughput! I would anticipate something in the range of 130-140 MB/s


user(a)ubuntu:~$ uname -a
Linux ubuntu.localnet 2.6.18-rc7 #2 SMP Wed Sep 13 11:28:41 CEST 2006 x86_64
GNU/Linux

-Christian
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