What do you do if the mirror fails, both disks die a horrible death or the place where your server stood, is empty on next morning?!
No mirror, complex raid array placed at different locations etc. could not convince me to take no mksysb. The mksysb is your last resort.
On paging space:
I don't know the official recommendation of the paging space today (RAM * 1,25?), but we usually have it somewhat small. On most boxes it's at least as large as the current RAM it has. When anything starts to swap, it doesn't matter if you have 4 GB paging space or 20 GB, because the box will be very slow and it has to be either tuned with vmo or needs more RAM, which has to be determined (or not).
For splitting paging space to SAN disks, I see no problem with this. We don't do that and keep it on internal mirrored disks, but currently I have no idea what should be bad about it... maybe a point of possible failure more.
Don't use SAN disk to create your paging space,if SAN disk error happen,or switch,or other device.all of device problem about SAN environment can cause your AIX hang.if you lost normal OS,you can't find root cause about problem.
i think the best way to do this is to have part of your paging on rootvg (memory*2) which is mirrored (or should be) then have an additional non-mirrored disk for additional paging space, even if it is on the san (which is still not a great idea)... for applications, etc - it is good to have nmon installed and running data out to a file so that you can review system data over a period of say a month - and see where your peaks/valleys are using nmon analyser (see ibm site)
Well... Let's look at an example: my server has 48GB of RAM so my paging space should be as big as 96GB?
Let's have a look at shockneck's top four tips about AIX paging space:
1) If AIX uses paging space you lack RAM or/and you configured vmo wrong. If tuning cannot help anymore get more RAM chips. Best thing is not to use paging space.
2) There is no such thing as an "official" recommendation about how big paging space should be (beside some application vendors such as SAP and/or your app's allocation policy) for many years now. Default settings for AIX have become pretty small since AIX 5L. Paging space shall be as big as necessary but as small as possible. I.e. it depends on how you use the server.
3) If AIX uses paging space it stores data that does not fit into RAM (and is not stored on disk already). Thus losing paging space is equivalent to losing a memory chip. Hence if you don't mirror it the other (SAN) storage should be at least as safe as it would be on mirrored disks.
4) If you use several paging spaces keep them at about the same size. If you want to you can make hd6 slightly bigger as it is used during boot. Furthermore speed of different disks used for paging should not differ to a large extend. Don't use two or more paging spaces on the same disk.