Re: raid1 performance

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doing a simple performance tests i obtained some very unexpected results: 

if

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i issue hdparm -t /dev/md2 i obtain 61 - 65 MB/s while issuing the same test 

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directly on the partitions which compose md2 (/dev/sda3 and /dev/sdb3) i
obtain 84 - 87 MB/s. I didn't expect a so big difference between md2 and 

one

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of its member. What can cause  this difference ? 

Maybe their read-ahead settings are different?
Check out "blockdev --getra /dev/md2", and compare that with the same
setting of the member disks. You can experiment with changing it by using
"--setra" as well.

Hi Roman,
thank you for your hint, I verified the read-ahead settings and they are the 
same for all the block devices involved in the test: the value is 256 for all 

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/dev/sd?? and for all /dev/md?
there should be something else which is influencing  raid 1 performance.
Have someone of you ever had a similar issue ?

Did you try:

# Set read-ahead.
echo "Setting read-ahead to 64 MiB for /dev/md3"
blockdev --setra 65536 /dev/md3

Hi,
i did some test modifying the readahead value with this results:

1) setting the ra value to 65536 only to the /dev/md2 has no effect on 
performance 

2) setting the ra value to 65536 to both /dev/md2 and on his member (/dev/sda3 
and /dev/sdb3) has REDUCED the performance of the single member (hdparm -t 
/dev/sd?3 produce value betwen 65 and 75 Mb/s) and has no effect on /dev/md2 (60 
-65 MB/s)

Onestly i'm confused...an increased read ahead value should normally increase 
performance on sequetial read, shouldn't it? 


Marco