Mirror consistency check for swap

[Tony Williams]

How much overhead is there when having WMC on primary swap with 32GB?

[A. Clay Stephenson]

Hi Tony:

When you begin to talk about performance overhead and swap those two don't go together well at all. If you need to worry about MWC then you are swapping way too much. The actual overhead is quite small but the real answer is not to swap at all. I can't imagine using a 32GB proimary swap space meaning you have really blown 64GB that you are not going to use if you've bought enough memory.

It usually makes much more sense to configure a much smaller primary swap (as little as 256MB but certainly no more than a few GB's) and configure other swap devices. If you are concerned about a dump area remember that you can configure separate dump and swap areas and there is no need (and actually a good reason not to) to mirror dump.

Nowadays, the old rules about configuring 2x-3x RAM (4x-6x with mirroring) for swap very seldom apply.

[Wodisch]

Hello Tony,

the answer is like "kind of none", as for a system with that much of swap (even if only intended) you have many GB of RAM, and then you simply have to go for "swapmem_on=1" in your kernel, and hence you'll only need swap space for at least 25% of your RAM...
And you have payed that much for the whole system, that you wuld not like the performance degrade of paging/swapping actually happening, and should have the money for more RAM, then!

Just my $0.02,
Wodisch

[Wim Rombauts]

Why would you need MWC on swap devices ?

MWC or mirror consistancy record is only intended for recovery after a crash to resynchronize the mirror. The swap space doesn't contain any usefull information when the system boots up, so there is no need to synchronize the swapspace.