Hello,
I've been going through the list archives to find a similar thread but
couldn't.
The setup:
debian lenny 2.6.26-1-amd64
raid1 array with 1 SSD disk and 1 regular SATA disk. It will be used for
a DB.
What I'm trying to achieve is to make the system read from the SSD disk
as long as the array is healthy and write delayed on the regular SATA
disk to benefit from the good random write performance of the SSD disk.
So far I've managed to achieve only the first goal - make the system
read from SSD on healthy array via the write-mostly option.
The problem is with the second option - write-behind. I've enabled it
and made two tests (one with healthy array and one with the SATA disk
marked as failed) with tiobench and the results are not good(1).
Generally with the healthy array I'm getting the write performance of
the SATA disk alone (in terms of requests/sec issued to the disk and
bytes/sec written). The SATA disk is obviously a bottleneck even with
the write-behind option set(2).
With a failed array (SSD active only) I'm getting the great write
performance of a SSD disk(3).
1. tiobench stats:
Unit information
================
File size = megabytes
Blk Size = bytes
Rate = megabytes per second
CPU% = percentage of CPU used during the test
Latency = milliseconds
Lat% = percent of requests that took longer than X seconds
CPU Eff = Rate divided by CPU% - throughput per cpu load
Healthy Array Random Writes (~4 MB/sec):
File Blk Num Avg
Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency
Latency >2s >10s Eff
---------------------------- ------ ----- --- ------ ------ ---------
----------- -------- -------- -----
2.6.26-1-amd64 2000 11264 1 4.34 0.687% 0.007
0.03 0.00000 0.00000 632
2.6.26-1-amd64 2000 11264 2 4.26 2.299% 0.012
0.04 0.00000 0.00000 185
2.6.26-1-amd64 2000 11264 4 4.12 6.018% 0.019
0.13 0.00000 0.00000 68
2.6.26-1-amd64 2000 11264 8 4.13 7.611% 0.020
8.94 0.00000 0.00000 54
SSD-Only Array Random Writes (~135 MB/s):
File Blk Num Avg
Maximum Lat% Lat% CPU
Identifier Size Size Thr Rate (CPU%) Latency
Latency >2s >10s Eff
---------------------------- ------ ----- --- ------ ------ ---------
----------- -------- -------- -----
2.6.26-1-amd64 2000 11264 1 138.12 21.85% 0.007
0.03 0.00000 0.00000 632
2.6.26-1-amd64 2000 11264 2 135.82 70.80% 0.012
0.05 0.00000 0.00000 192
2.6.26-1-amd64 2000 11264 4 135.10 176.0% 0.018
0.07 0.00000 0.00000 77
2.6.26-1-amd64 2000 11264 8 145.38 380.3% 0.020
3.45 0.00000 0.00000 38
2. healthy array iostat output (sda1 being the SSD disk):
Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s
avgrq-sz avgqu-sz await svctm %util
sda1 0.00 14183.50 0.00 220.50 0.00 56.27
522.59 5.23 23.70 1.25 27.60
sdc3 0.00 14272.00 0.00 129.50 0.00 56.99
901.22 137.77 1108.25 7.72 100.00
3. ssd-only array iostat output:
Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s
avgrq-sz avgqu-sz await svctm %util
sda1 0.00 51941.50 0.00 483.00 0.00 209.70
889.16 140.81 282.74 2.07 100.00
sdc3 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00
So the questions is How behind can write-behind write? And can we get a
better performance in a similar setup.
thanks upfront!
--
regards,
Georgi Alexandrov
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