On Wed, 2005-07-13 at 19:02 +0100, David Greaves wrote:
> 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)
this is so interesting to see! seems that some read ahead parameters
have negative impact.
>
> 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
>
-
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
