>>
>>
>> >> doing a simple performance tests i obtained some very unexpected results:
>if
>> >> i issue hdparm -t /dev/md2 i obtain 61 - 65 MB/s while issuing the same test
>>
>> >> 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
>> >> 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
>> /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
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