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[linux-lvm] Re: optimum blockdev --setra settings



John McMonagle wrote:

Anyone establish optimum blockdev --setra settings for raid on a 2.6 kernel?

yep. I found 0 at all levels except the topmost. ie /dev/hdx -> ra=0 /dev/mdx -> ra=0 /dev/lvm/ -> ra=xxxx


There has been some discussions on the lvm mailing list.

must have missed it...
Here's the message I cc'ed the lvm group *last* time this came up (12 Dec '04) ;)


I hacked up a quick script to test permutations of readahead - it's not exactly bonnie+++ but it may be useful.
I wish I'd bothered with mdadm stripe sizes too - but the array is pretty full now and I'll live with what it delivers.


Essentially I found the best performance on *my* system with all low level devices and the md device set to a 0 readahead and the lvm device set to 4096.
I'm only interested in video streaming big (1+Gb) files. Your needs (and hence test) may differ.


my system is 2.6.10-rc2, xfs, lvm2, raid5, sata disks.

cc'ed the lvm group since this often seems to come up in conjunction with you guys :)

For your entertainment...

#!/bin/bash
RAW_DEVS="/dev/sda /dev/sdb /dev/sdc /dev/sdd /dev/sde /dev/sdf /dev/hdb"
MD_DEVS=/dev/md0
LV_DEVS=/dev/huge_vg/huge_lv

LV_RAS="0 128 256 1024 4096 8192"
MD_RAS="0 128 256 1024 4096 8192"
RAW_RAS="0 128 256 1024 4096 8192"

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

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

function show_performance()
{
COUNT=4000000
dd if=/dev/huge_vg/huge_lv of=/dev/null count=$COUNT 2>&1 | grep seconds
}

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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