[CentOS] Swap Considerations

[Ross S. W. Walker]

> -----Original Message-----
> From: centos-bounces at centos.org 
> [mailto:centos-bounces at centos.org] On Behalf Of DamianS
> Sent: Tuesday, February 27, 2007 1:59 AM
> To: CentOS mailing list
> Subject: Re: [CentOS] Swap Considerations
> 
> On Tue, 2007-02-27 at 07:36 +0200, Alvin Chang wrote:
> > On 27/02/07, Feizhou <feizhou at graffiti.net> wrote:
> > > When I get firefox and thunderbird fired up, memory usage 
> can go through
> > > the roof so I have allocated 2GB (maximum swap partition 
> size btw) for
> > Pardon me, but I just set up a CentOS box with 8GB of swap partition
> > yesterday. Am I misisng something?
> 
> Not really. 2GB is (or was) the maximum _safe_ swap partition size.
> However, I have a system with a 4GB swap part and one with a 3GB swap
> running CentOS, and they both run fine.
> 
> I have heard however, that smaller-sized swap partitions are 
> faster/more
> efficient. ie. 3x1 GB swap partitions are 'supposed to be' 
> better than 1
> x 3 GB partition.
> So when I'm building new systems, I create a number of 1 GB swap
> partitions depending upon predicted load, and give them differing
> priorities.
> 
> I've theorized (but never bothered to do some benchmarking or to put
> into practice) that optimum swap strategy may be to have varying sizes
> eg. 512 MB with highest priority, then 1 GB, 2 GB and 4GB with lowest
> priority.
> But I think that would be 'gilding the lily', for just a 'theoretical'
> increase in eficiency, as opposed to making a practical difference.

If you have 4 1GB swap partitions of equal priority it will interleave
operations between them giving you a RAID0 type effect and increasing
overall swap performance.

If you setup swap on top of RAID-1, say with LVM, then this scenario
will give you the best performance and be able to continue running in
the event of a drive failure.

-Ross

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