LVM pe_size large or small ?

[Tim Nelson]

After reading and reading I just cannot decide which is better. A large pe extent size or a small. Here is the scenario. 9GB jamaica F/W scsi disks. Using extent based striping with mirrors. JFS filesystems. File sizes are all 2GB databases. Does anyone know exactly what a small 1 MB pe_size would get me. WHat about a 256MB pe_size ? I am leaning toward the fact that the large pe size would undermine the attempt to spread the i/os across the disks because the chunks per disk are very large. I assume the default of 4MB per extent is there for a reason but I have a DB vendor that is suggesting a 32 MB extent size and I am having a hard time proving or disproving its benefits.
Anyone have any opinions ? Sorry if I left any details out.

[Paula J Frazer-Campbell · Accepted solution]

Hi
Why is the DB vender requesting 32 MM has he done any testing and if so in what environment.

My own system runs 2MB extents and handles 1 Gb files with no probs.

To really find out the best you need to try and measure on various settings.

[John Palmer]

Hi Tim,

With extent based striping you are better off with the smallest PE size that you can get. This is 1Mb with your 9Gb disks but increases with larger disks because of the .

The smaller the extent size, the more you will even out I/O requests over the disks that make up the stripe set. The larger then the more likely that you will have 'hot' areas on a particular disk.

I don't know of any benefits that a 32Mb extent size has over 4Mb other than less output from lvdisplay -v. It does have the disadvantage that the minimum size volume is 32Mb times the number of disks in your stripe set. This is probably not a problem for filesystems but can be an issue if you want to create small raw volumes.

Regards,
John

[Alan Riggs]

The only advantage I know of for increasing the extent size is for very large VGs. There is a "hard-coded" limit to the number of physical extents which can be addressed within a single volume group (64K, IIRC). This limit is not escapable. Therefore, to create a volume group with a size larger than 256 GB you need to increase the extent size from the default of 4 MB.

[Tim Nelson]

John's was the input I was looking for. I have learned not to trust vendors recommendations but to take them into consideration when making my own decisions. I was already leaning towards the 1MB per extent size as it made the most sense.
Thanks to all!!