Dan Christensen wrote:
>Ming Zhang <mingz@xxxxxxxxxxx> writes:
>
>
>
>>test on a production environment is too dangerous. :P
>>and many benchmark tool u can not perform as well.
>>
>>
>
>Well, I put "production" in quotes because this is just a home mythtv
>box. :-) So there are plenty of times when it is idle and I can do
>benchmarks. But I can't erase the hard drives in my tests.
>
>
Me too.
>>LVM overhead is small, but file system overhead is hard to say.
>>
>>
>I expected LVM overhead to be small, but in my tests it is very high.
>I plan to discuss this on the lvm mailing list after I've got the RAID
>working as well as possible, but as an example:
>
>Streaming reads using dd to /dev/null:
>
>component partitions, e.g. /dev/sda7: 58MB/s
>raid device /dev/md2: 59MB/s
>lvm device /dev/main/media: 34MB/s
>
>
This is not my experience.
What are the readahead settings?
I found significant variation in performance by varying the readahead at
raw, md and lvm device level
In my setup I get
component partitions, e.g. /dev/sda7: 39MB/s
raid device /dev/md2: 31MB/s
lvm device /dev/main/media: 53MB/s
(oldish system - but note that lvm device is *much* faster)
For your entertainment you may like to try this to 'tune' your readahead
- it's OK to use so long as you're not recording:
(FYI I find that setting readahead to 0 on all devices and 4096 on the
lvm device gets me the best performance - which makes sense if you think
about it...)
#!/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 $dev
done
}
function show_performance()
{
COUNT=4000000
dd if=$LV_DEVS 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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