[mythtv-users] Raid 5 Performance Tweaking

[Dean Wilson]

Sorry if this is a stupid (perhaps related?) question:  I have LVM
installed on top of my raid array.  Should my physical extents
(defined in the  as "chunks of data") be the same size as the raid
chunks?  (vgdisplay /dev/VolGroup00 returns a PE size of 32MB.)

    --- Volume group ---
    VG Name               VolGroup00
    System ID
    Format                lvm2
    Metadata Areas        1
    Metadata Sequence No  6
    VG Access             read/write
    VG Status             resizable
    MAX LV                0
    Cur LV                5
    Open LV               5
    Max PV                0
    Cur PV                1
    Act PV                1
    VG Size               931.53 GB
    PE Size               32.00 MB
    Total PE              29809
    Alloc PE / Size       26935 / 841.72 GB
    Free  PE / Size       2874 / 89.81 GB
    VG UUID               qeWJuY-j7MI-6R6n-UETw-5LyN-knCS-5Um8Ay

Thanks!

On 6/5/06, Dean Wilson <dean.k.wilson at gmail.com> wrote:
> I recently installed Fedora Core 5 from scratch on a raid5 array of 5
> disks.  But I didn't see any options for how to configure the raid
> array, and now I'm wondering if it could be tweaked better for large
> files (i.e. for use with myth)  I read in the wiki
> (http://www.mythtv.org/wiki/index.php/RAID, and
> http://www.tldp.org/HOWTO/Software-RAID-HOWTO-9.html) that a 32K chunk
> size and 4K block size was the best setup.  However, I notice that
> when I run 'cat /proc/mdstat' I get the following:
>
>      Personalities : [raid5] [raid4]
>      md0 : active raid5 sdd1[4] sdc1[3] sdb1[2] sda1[1] hda3[0]
>            976783360 blocks level 5, 256k chunk, algorithm 2 [5/5] [UUUUU]
>
> Apparently the default setup for FC5 was 256k chunks.  (Which doesn't
> state block size -- anyone know how to find that?)  I noticed that the
> raid performance page doesn't test beyond 32K chunk sizes -- does
> anyone know which is best?  (I would think that larger chunk sizes are
> better for large files, but I don't know...)
>
> Also, I found the Raid Howto
> (http://www.tldp.org/HOWTO/Software-RAID-HOWTO-5.html) which states
> that "RAID-{4,5} performance is severely influenced by" stride
> settings for mke2fs.  Since I'm using xfs, is there a similar setting
> I should use?
>
> (I guess I should also ask if it's possible to change these settings
> at this point?  If not, would the performance gain be worth
> reinstalling?)
>
> Thanks in advance,
> ~Dean
>