LVM Striping and Mirroring

[Chris Germany]

I would be very interested in knowing whether it is possible to set up an array of FC1010D disks that are both stripped and mirrored (RAID 1/0) with a strip size less than 1MB. I would like to set the disks up with a stripe width of 32K or 64K for an Oracle database OLTP application. The large strip width of 1Mb I know is going to cause a slow down in the write rate but I am being told by my system administrator that this will not be possible.

[CHRIS_ANORUO]

Hello,

There is no need for you to do LVM stripping on a RAID system.
You acn do LVM mirroring is the disk space available.

http://www.hp.com/cposupport/manindex/hpsurestor19162_eng_man.html

[Stefan Farrelly]

It certainly is possible. You need to use the -D distributed option on lvcreate (along with -i and -I to set the stripe size to 32k)

[James R. Ferguson]

Hi Chris:

You can mirror and strip with "extent-based mirrored strips". A good discussion can acutally be found in the man pages for 'lvcreate'. You can adjust the extent size on the disk during the 'pvcreate' if you wish. 'lvcreate's distributed allocation policy is incompatible with the striped scheduling policy ('-i stripes'). A detailed example appears at the end of the 'lvcreate' man pages.

...JRF...

[Chris Germany]

The information Stefan and James have provided has helped but does not answer the question of whether HP supports 32K stripes that are mirrored on FC1010D disks, these are not RAID devices such as the AutoRaid.

As James and the man pages for lvcreate state, the distributed allocation policy -D is incompatible with striped scheduling -i. Thus Stefans lvcreate -D -i -I 32K is not supported. Can I ask whether the striped scheduling can be used with the -m option for mirror copies. Thus the command would be somnething like:
lvcreate -m2 -i4 -I 32K /dev/xyz

Would this give me a mirroed set of disks stripped at 32K over 4 disks ?

I would like to know the answer because when we have used the Distributed allocation with the smallest of a 1Mb physical extent. Subsequent performances tests show alot of write disk activity which would be aided by a smaller stripe width.

[Stefan Farrelly · Accepted solution]

Sorry, my mistake. The -D distributed option does allow striping and mirroring, but only at the extent size - which at a minimum can be 1 MB, no smaller. The -i option supports right down to 4k, but you cant use this with LVM mirroring, or extent-based manual striping (in James' reply).

So your options are;
1. Purchase and install VxVM (instead of LVM/JFS). This does allow what you want - it even allows software based RAID (no hardware needed!).
2. Use a 1MB minmum stripe size
3. Use the -i stripe size down to 4k but no mirroring - any disk protection will have to be done at the hardware level (Autoraid, EMC, XP256, Nike etc.)

HP use 1MB extent based striping and mirroring on all their internal Openmail servers and the performance is brilliant - we tried no lvm mirroring and striping down to < 1 MB but found no noticeable improvement. We use lots of buffer cache and our external disk arrays have lots of cache which we weighted towards writes (not reads) - so this offset and writing degradation with using 1MB stripes as you suggested with your application.

[James R. Ferguson]

Chris:

With regard to the Veritas Volume Manager (VxVM), if you haven't taken a look at this document, you might want to do so:

"VERITAS Volume Manager 3.1 Migration Guide"

http://docs.hp.com/hpux/onlinedocs/B7961-90001/B7961-90001.html

...JRF...