Originally posted by: abdurixit

I have a server which needs about 100GB paging space (sounds crazy, but true).
rootvg is mirrored over 2 x 146GB local SCSI disks. I have ordered 5 x 20GB SAN disks.
If I understand correctly, paging space allocation should follow following rules:

• one paging space on one disk
• all paging spaces should have same size

If I follow these rules, then I have to create 4 x 20GB paging spaces on 4 SAN disks and increase hd6 to 20GB, right?

But I have fear that the os disks become bottleneck when there is a high paging space usage.
SO I decided to leave hd6 with 512MB and create 5 x 20GB paging spaces with 5 SAN disks. Are there any arguments speaking against it? Previously, I have configured a server similarly and did not see any obvious problem.

Originally posted by: orphy

100G paging! Interesting... The way you talked, it sounded like you have no choice but I'm just curious as to why not more memory? No money?

There's been a few school of thoughts out there throughout the years as to how to cut the LVs for paging but I usually stick with the thought of 1 paging per disk/even size/spread across as many disks as possible. I sometimes add an extra PP to hd6 since they said it's good for you due to the OS overhead but I never really care for proving it.

Check these out and make your choice.

http://www.ibm.com/developerworks/wikis/display/WikiPtype/Paging+Space
http://www-03.ibm.com/systems/power/software/aix/whitepapers/pagingspace.html
http://www.redbooks.ibm.com/abstracts/sg246039.html
Orphy

Originally posted by: SystemAdmin

Also, you might want to review who's saying you "need" the space.

I find lots of DBAs, especially Oracle, say you need 2-3x the physical memory per Oracle's recommendation. However, if they set everything up right, they really don't need it. If you have 48Gb of physical, you probably need 12Gb tops in paging.

Start with a good multiplier like that 20Gb - give them 1 to start. Wait until they start using 90% of that before you give them another one. I've never seen an AIX box use 100Gb of paging.

Food for thought.

Originally posted by: Ayaz_Anjum

If you want to put paging space on LUNs from SAN, some considerations ...

LUN from SAN is logical volume spanning already on multiple physical disks. So spreading swap across multiple LUNs off san storage does not mean much since already each LUN is split across multiple physical disks

thanks

Originally posted by: abdurixit

Thanks for your responses.
I guess I know what I am talking about. Actually I have about 20+ DB instances and 10-15 of them running concurrently on the partition. I have allocated 160GB RAM, but still I have to create such a big paging space to handle the worst case scenario if mistakenly all of them activated, it can happen and crash my server, I had it once already.
My question was actually not about the paging space size or how the LUNs on storage box configured ( we have Raid 7+1 and each 20GB is striped over 28+4 disks).
My actual question is how do I make a 100GB paging space.

a. 100GB = 512M hd6 + 5 x 20GB PGSPs on 5 SAN Disks (I know, it is 100.5 GB)
b. 100GB = 20GB hd6 + 4 x 20GB PGSPs on 4 SAN Disks.

I do not like (b.) because of high I/O on OS disk if there is a extreme last in paging space.
That is why I prefer to choose (a.), but the IBM Wiki page tells me it is wrong.
What is your recommendation?

Originally posted by: orphy

I don't think the wiki says you are wrong. All those paragraphs are nothing but guidelines/recommendations. Personally, I've been to many sites throughout the years and have seen many different ways of dealing with paging spaces but I wouldn't necessarily say they are wrong (well, maybe a few of them!) so I typically just tell them to stick with the recommendations.

Anyway, if you think a) will better serve you, go for it. You seen to know what you are doing and if the choice doesn't work too well for you, you can also try b). Just look at this as part of tuning your box. You should know that a lot of choices in tuning is neither right or wrong. Sometimes, you just have to give your best guess.
Orphy