Re: [Xen-devel] Optimizing NFS to Xen

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I read about NFS in this mail (at kernel-list): [PATCH 00/33] Swap over NFS
> -v14
>
> This is outdated information? I trying to understand why i can`t use SWAP
> with NFS.

Swap with NFS has not been allowed for technical reasons.  I think that 2.6.24 
might have patches to allow swapping over NFS, but I'm not really sure.  
There's certainly an effort in progress to get swap-over-NFS into mainline 
Linux in the not-too-distant future.

Patches to allow it have been floating around for a while, so you could 
probably find one and apply it to your kernel.  Actually, I thought that 
debian were including a patch for this in their kernel at one stage but I'm 
not sure...

Anyhow, the basic problem with swapping over NFS is: you want to swap when 
you're low on memory; you need to allocate memory in order to do network IO; 
how do you swap over the network if you've got no memory to allocate for 
network buffers?

I'm not sure how previous patches address this (or how good they were!), but I 
understand the latest round of patches for current kernels keep some memory 
in reserve in order to prevent a lack of memory freezing up the system and 
preventing forward progress.

The problems (and solutions) here are basically the same whether you're 
running on a real machine or under Xen, so there's not really anything 
Xen-specific going on.

Cheers,
Mark

> Regards,
> Luiz Vitor Martinez Cardoso aka Grabber.
>
> On Nov 3, 2007 6:08 PM, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> wrote:
> > > fast network help. If you use IDE disks, use hdparam to tune them for
> > > optimal transfer rates. If you
> > > support multiple, simultaneous users, consider paying for SCSI disks;
> >
> > SCSI
> >
> > > can schedule multiple,
> > > interleaved requests much more intelligently than IDE can.
> >
> > Thats rather outdated documentation



-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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