Re: [Xen-devel] Optimizing NFS to Xen

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> Thanks for reply!
>
> I'm thinking... is really necessary use SWAP over ethernet (using NFS)? The
> RAM memory have highest transfers rates and access time, why we need
> migrate information (pages from ram) accross a network? It is stupid and
> decrease a lot o performance???
>
> Can anyone justify it?

Well, just swapping to disk can cost a significant amount of performance 
because the /latency/ of accessing a disk is way more than that to access 
RAM.  The biggest two arguments for swapping to disk I can think of are:

a) move unused data out of RAM to make more space for caches and other useful 
stuff
b) avoid out of memory conditions if you're running programs that use close to 
(or more than) your total available RAM

The same arguments basically apply to swapping over NFS.  You might then ask 
why we'd choose to swap over NFS instead of directly to disk, given using a 
disk directly might perform better.  The big win here is that it makes it 
possible to have diskless machines which are still able to swap.  With 
virtual machines this could be very handy because:

a) it gives you a convenient central point (the NAS filer) to administer 
virtual machine filesystems.
b) virtual machines can still access their filesystem and swap data after 
being live migrated to another physical host

Cheers,
Mark


> Regards,
> Luiz Vitor Martinez Cardoso aka Grabber.
>
> On 12/2/07, Mark Williamson <mark.williamson@xxxxxxxxxxxx> wrote:
> > > 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 This is outdated information?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!



-- 
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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