topic Re: LVM Extent Based Striping in Operating System - HP-UX

LVM Extent Based Striping

Richard Jenkins_1 — Wed, 11 Jan 2006 11:06:59 GMT

Hi

I am running HPUX 11.11 and Oracle 9.2i. I would like to stripe and mirror my data with a 64k stripe width, but as I cannot then mirror the data, I am obliged to use 'extent based striping' which does support mirroring. The minimum stripe size/extent size for this is 1Mb. I do not have enough disks to dedicate some for just this purpose, so when I create the volume group with 1Mb extents, the 1Mb extents will be used by other (probably Oracle) files - even if they themselves are not striped. So... Is there a down-side to using a 1Mb extent stripe size for Oracle data? And is there a down-side to using a 1Mb extent size for other (non-striped) data?

Any help will be welcome.

Richard.

Re: LVM Extent Based Striping

Victor BERRIDGE — Wed, 11 Jan 2006 11:24:48 GMT

Hi Richard,

To be able to answer we would have to know what kind of disks you are using...

I just did something of the sort this week-end but with the default PE size of 4MB, to explain, I had to migrate for we are changing bays (HDS9980V to HDStagmastore) and switches etc. As I were on LVM stiped disk I could not miror, so the only alternative left was to use extend striping on the new PVs.
I have noticed a better performance in big transfer (well its over 6 disks per vg now...)

All the best
Victor

Re: LVM Extent Based Striping

Bernhard Mueller — Wed, 11 Jan 2006 11:39:34 GMT

Richard,

you need to take into account the LVM limits:
http://docs.hp.com/en/6054/Limits_wp.htm

So typically this is used with JBODs and there you have to take the disk size into account. e.g. if you have 36GB disks you can create a VG with 14 disks if you use 2MB extend size: "vgcreate -s 2 â e 17600 â p 14 ..."

I do not quite understand what you mean with "if they themselves are not striped.."

Regards,
Bernhard

Re: LVM Extent Based Striping

A. Clay Stephenson — Wed, 11 Jan 2006 11:49:38 GMT

There is no downside to using 1MB PE's but it does severely limit the maximum size of a physical volume because the maximum size of any PV is then 64K * 1MB.

I have yet to see any significant throughput gains to be had from extent-based striping because the smallest possible extent (1MB) is still much too large to be a good stripe size to distribute the i/o.

Re: LVM Extent Based Striping

Richard Jenkins_1 — Thu, 12 Jan 2006 04:34:20 GMT

Hi

Thanks for the responses. Here are the answers to your questions and some more information to further explain my situation. The disks are HP 36.4G ST336754LC. I would probably create a 3 disk stripe set. The Oracle data tablespace comprises 7 500Mb datafiles, of which 4 are much busier than the others. By putting these files in a striped volume I hoped to more evenly spread the I/O. When I said 'if they themselves are not striped' I was referring to creating a 3 * 36Gb volume group with 1Mb extents and only using perhaps 10Gb for my striped Oracle data volume. That would leave 98Gb of 1 Mb extents to use for other volumes. It may be that I create those volumes as unstriped; although on reflection I can't think of any reason not to create them as striped volumes as well.

Cheers,

Richard.

Re: LVM Extent Based Striping

Victor BERRIDGE — Thu, 12 Jan 2006 09:50:08 GMT

Hi again,

Should I understand you have only 3 PV? I dont see how you will be doing the mirroring after striped over the 3 disks...
Or you do have 2* 3 disks, in which case I just like you see no reason why not stripe all your logical volumes then mirror the whole group
You should have better performance...
If you have 2 controllers I would try to alternate the pvs and see to create alternate paths...

All the best
Victor