Performance issues with PE size larger than 4Mb?

[thewho?]

Hello,
Does anybody know how much is performance affected by changing the PE size to a value different to the default 4Mb?
I'm playing with large disks and got the "file too large" error. I increased the -s to 8, and the problem went away but I'm wondering if this would affect the performance.
Would it be better to decrease the -p?
Thanks in Advance

[Sachin Patel]

Hi Luis,
It depends on your type of usage. If you are accessing large number of files and they all are large in size then your PE > 4mb will increase your performance. If you have files that are small in size then PE > 4mb will decrease your performace.


Sachin

[A. Clay Stephenson]

Hi Luis:

No real performance implications are associated with this. What does have an impact is logical volume stripe size (and the number of devices you are stripped across) but PE size has none.

Clay

[James R. Ferguson]

Hi Luis:

The problem you encountered is a reflection of the fact that the LVM tables must fit into one disk extent. A failure of this requirement gives the 'vgcreate' "file too large" error. You can reduce the values of either or both 'max_pv' and 'max_lv' and/or increase the value of 'pe_size'.

Remember that once established, the values of 'max_pv', 'max_lv' and 'pe_size' cannot be changed, so choose carefully. This will impact your options when adding addditional disks, creating new logical volumes and extending existing logical volume extents onto new disks later.

...JRF...

[MANOJ SRIVASTAVA]

Hi Luis

How do you have your disks laid . Incase you are on differnt controlleres then definately the increase should slow down the performance as logically to read that size of data it woul spend time switching on between them , however in case it is on the same controller then i dont think there would be any degrade . This is also a combination of many factors like what database you are running , do you read the files using commnads like dd , cpio ( for backups ) etc etc.

Manoj Srivastava

[MANOJ SRIVASTAVA]

Louis

One more thing if you have 4 disks in a volume group and they are across 4 controllers then the policy of PE ( physical extensts 4MB ) and each strict policy will give a better performance thatn PE = 8 MB .

Manoj Srivastava

[Wodisch]

Hello Luis,

you can have PEs up to 256MB currently, and all you
get is propably a samll performance boost, not a slow
down! (for with bigger PEs you need less of them,
which is less work for LVM/kernel - which is always a
good idea).

HTH,
Wodisch

[A. Clay Stephenson]

Hi Luis:

I suppose I'll be a physicist here and tell you the correct answer is to measure for yourself. Having said that I will be very surprised if you see any significant and repeatable differences.

Regards, Clay