Which PP-size to choose?

[khalidaaa]

Hi all,

maybe is a silly question but i will ask it any way.

i'm copying data from an old server to a new one

in the old server the vg was 256 in size but the new one is 16 Meg

which is better to use?

regards
Khalid

 

[hirschaj]

What size are the disks? PP-size is really determined by disk size.


Jim Hirschauer

http://www.aixexpert.com

 

[Breslau]

depends on the average filesize you expect to have in the FS. with the later versions of AIX, the pp to disk size limitations have pretty much gone away, ie a large LUN or drive can have a smaller pp/lp size.

 

[costiles]

Generally speaking, if you are striping and you want to spread your file over more than one disk so that more than one person can write to a file and consquently multiple disks at one time, then you want the pp size to be small enough so that multiple users can access the data on more than one disk. With the T factor in the setup of the volume group, you can make your pp size almost anything that you want - but the smaller the pp size the fewer number of disks that can be included in your volume group. If you are not striping - ie - the data fills one disk and then moves to the next disk only when the first is filled, then I don't think pp size will matter.

 

[khalidaaa]

Ok let me explain the situation over here

I'm using SAN D4500 which is configured as RAID 10 (Stripping+Mirroring)

Each disk is 73G and i'm using two disks for the stip and two more disks as a mirror for the formers.

The stip is splitted into 10 Logical drives (each approx. 13.5)

For this server (actually its an LPAR), i've allocated 6 logical drives.

now when i did the copying from the old server to the new there has been some differences in the sizes of the filesystems!!!

edms:/image2   114294784   9363360   92%   243428     2% /image2
/dev/image1t   114294784  14802704   88%   243415    13% /image1t

those are the two filesystems but as you can see the second (which is the new) is less in space than the old

so am i utilizing the space by making it 16M instead of 254M???

Thanks guys for the excelent support

Regards
Khalid

 

[mrn]

From a space point of view - Large files; use a large PP. Lots of small files use a smaller PP

If you have say a filesysytem of 256Mb with a PP size of 256 and you want to grow the filesystem by 100Mb you would be effectivly losing 156Mb.

A bigger pp-size might lead to bigger spaces between pps (can be resolved with migratepp) but with a bigger PP size there is also a bigger chance that a file fits into a PP.

Another impact is striping ("poor mans stripe"); the smaller the PP size is the more a lv gets distributed over several disks has the same pro+cons as the striping size in "real" striping

However that there are so many io-related options that impact performance that the pp-size doesnt make a big difference i always choose the ppsize so that i loose as little space as possible.

Mike

"A foolproof method for sculpting an elephant: first, get a huge block of marble, then you chip away everything that doesn't look like an elephant."

http://www.airport-parking-site.co.uk/