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LVM Extent Based Striping

Richard Jenkins_1
Occasional Contributor

LVM Extent Based Striping

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.
5 REPLIES
Victor BERRIDGE
Honored Contributor

Re: LVM Extent Based Striping

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
Bernhard Mueller
Honored Contributor

Re: LVM Extent Based Striping

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
A. Clay Stephenson
Acclaimed Contributor

Re: LVM Extent Based Striping

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.
If it ain't broke, I can fix that.
Richard Jenkins_1
Occasional Contributor

Re: LVM Extent Based Striping

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.
Victor BERRIDGE
Honored Contributor

Re: LVM Extent Based Striping

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