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RE: Re: [Xen-users] Swap: create or not?

  • To: George Shuklin <george.shuklin@xxxxxxxxx>, <xen-users@xxxxxxxxxxxxxxxxxxx>
  • From: Jeff Sturm <jeff.sturm@xxxxxxxxxx>
  • Date: Mon, 14 Feb 2011 09:53:36 -0500
  • Cc:
  • Delivery-date: Mon, 14 Feb 2011 06:54:32 -0800
  • List-id: Xen user discussion <xen-users.lists.xensource.com>
  • Thread-index: AcvLIbOgHqyJ17DdSWGdFsawRw+ZnQBNCTWA
  • Thread-topic: Re: [Xen-users] Swap: create or not?

The OOM killer will run if you exhaust memory without swap, sure, but the same is the case if you exhaust swap.


Your point about I/O is an interesting oneâwe'd avoided using swap on domU machines simply to keep I/O away from our shared storage. But if you have measurements showing that swap decreases I/O during steady-state operation due to increase buffer cache utilization, then swap is clearly working well for you.


I'm just catching up on this thread and intrigued by the arguments both ways. Seems like there's no single right answer here.




From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of George Shuklin
Sent: Saturday, February 12, 2011 8:59 PM
To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Fwd: Re: [Xen-users] Swap: create or not?


Many or not - it about 30-100Mb per VM, wich coming out as about 40Gb of total (saved) memory for all guest VMs. This memory used as buffers/page cache and effectively reduce IO load on shared storage.

Second very important usage: when user processes starts to eat memory, it is some lag until we add more memory. Of couse every VM have some free memory, but in case it was not enough - swap will save guest VM from been OOMkilled.

So I strongly supports for domU swap.

-------- Original Message --------


Re: [Xen-users] Swap: create or not?


Fri, 11 Feb 2011 12:38:50 +0100


Henrik Langos <hlangos-xen@xxxxxxxxxxxxxx>


George Shuklin <george.shuklin@xxxxxxxxx>




I'd argue against swap in domU if you can avoid it.
If your application needs that much more RAM you'll be better off
giving that VM more RAM instead.
If your application doesn't give a damn about memory usage, it 
will be the first on the oom-killer's menu. -> Problem solved. ;-)
If you allow the application to go deeply into swap, it 
will place some heavy IO load on your machine and thus bog down 
- every other application on that domU AND 
- every other domU including their applications AND
- your dom0 (very nasty if you need your dom0 to do IO to a SAN).
So, I'd say avoid swap if you can, and if you can't than keep it
to a minimum (lets say 256M).
About balooning: AFAIK this only means taking memory away from dom0.
DomU's have a fixed amount of memory, don't they?
Balooning for dom0?
I'd say: Avoid it like the plague! It will bog down your dom0 horribly
and the only reason I can see for using it, would be on a desktop where
ocassionally you use a xen domU.
For a server I strongly suggest following the best practices:
On Fri, Feb 11, 2011 at 02:18:48PM +0300, George Shuklin wrote:
> Yes, swap is almost mandatory - it will save machine from state 'very
> low mem'. From my research (mostly on XCP) this state may cause
> unstability for some kernels, causing OOM_killer to kill many processes
> (sometimes including init - means kernel panic). Swap reducing chances
> to get to this state.
> Second very important point: ballooning. When ballooning (inflating
> balloon = reduce mem) occur, if kernel have no free address space it can
> invoke OOM_killer. Swap will save in case slightly wrong values for
> balloon.
> Ð ÐÑ., 11/02/2011 Ð 10:12 +0000, Mauro ÐÐÑÐÑ:
> > When creating a DomU I've always set a swap.
> > Do you think it is necessary to have a swap in the DomU?
> > 
> > _______________________________________________
> > Xen-users mailing list
> > Xen-users@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-users
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