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Re: [linux-lvm] Performance of striping



Hans-Joachim Baader wrote:
Hi,



snip

Have you tried fiddling with read ahead ?
"blockdev --setra ..." on the striped LV.

read ahead is currently 256 on all devices. Thanks for the hint.
I will try some other values. Any Idea what is recommended?

I originally found this script to test read ahead settings on some email list - I forget where or who.

#!/bin/bash

RAW_DEVS="/dev/sdb /dev/sdc /dev/sdd /dev/sde"
MD_DEVS=/dev/md17
LV_DEVS=/dev/mapper/vg17-home

LV_RAS="128 256 1024 4096 8192"
MD_RAS="128 256 1024 4096 8192"
RAW_RAS="128 256 1024 4096 8192"

log=/tmp/ra_test.log
rm -f $log

function show_ra()
{
for i in $RAW_DEVS $MD_DEVS $LV_DEVS
do echo -n "$i `blockdev --getra $i`  ::  " | tee $log
echo ""  | tee -a $log
done
}

function set_ra()
{
RA=$1
shift
for dev in $@
do
 blockdev --setra $RA $dev
done
}

function show_performance()
{
COUNT=4000000
/usr/bin/time dd if=$LV_DEVS of=/dev/null count=$COUNT 2>&1 | tee -a $log
echo "<><><>" | tee -a $log
}

for RAW_RA in $RAW_RAS
do
set_ra $RAW_RA $RAW_DEVS
for MD_RA in $MD_RAS
 do
 set_ra $MD_RA $MD_DEVS
 for LV_RA in $LV_RAS
   do
   set_ra $LV_RA $LV_DEVS
   show_ra
   show_performance
   done
 done
done


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