Re: [Xen-users] Purpose of mem-max command

[Adam Goryachev <mailinglists@xxxxxxxxxxxxxxxxxxxxxx>]

On 11/10/15 21:45, Marco Guazzone wrote:
> On Sat, Oct 10, 2015 at 6:36 PM, Simon Hobson <linux@xxxxxxxxxxxxxxxx> wrote:
> > Marco Guazzone <marco.guazzone@xxxxxxxxx> wrote:
> >
> > > I don't understand the behavior of the "xl mem-max" command.
> > >
> > > The help says: "Set the maximum amount reservation for a domain."
> > > But if I try to use it I see no effect on the target domain.
> > >
> > > It seems that only the "xl mem-set" command has effect on the VM.
> > That sounds correct. mem-max sets the maximum that mem-set can configure. I 
> > can't see that it's that useful from the command line, but I have mem-max set 
> > is several of my config files where I want to start a machine with a configured 
> > amount of ram, but have the option to increase it later (typically because I've 
> > had to cut some machines down to shoehorn another VM onto the host)
> Hi Simon,
>
> What I don't understand is how to use it effectively.
> Could you provide a practical example of how to use it?
>
> Let me show you some of tests I've done (sorry for the length of the
> message). Before performing a test, I always reboot the VM to cancel
> the effects of previous tests
>
> My CentOS 7 VM, called 'memtest', is statically configured (in the
> config file) to have 8GB of memory:
>    maxmem = 8192
>    memory = 8192
> Indeed, once booted I get
> $ xl list
>    Name                                        ID   Mem VCPUs    State    
> Time(s)
>    Domain-0                                     0 71845    16
> r-----   12630.1
>    memtest                                     35  7935     4
> -b----      10.0
>
> and inside the VM
>
> [root@memtest ~]# free -h
>                  total        used        free      shared  buff/cache
>   available
>    Mem:           7.5G        106M        7.4G        8.3M         89M
>        7.3G
>    Swap:          1.0G          0B        1.0G
>
>
> - TEST #1
> $ xl mem-max memtest 3g
> libxl: error: libxl.c:3916:libxl_domain_setmaxmem: memory_static_max
> must be greater than or or equal to memory_dynamic_max
> : Success
> cannot set domid 35 static max memory to : 3g
Try this test, in the config file:

  maxmem = 20480
  memory = 8192


When you start the VM, it should have 8G

Then, you can allocate some extra RAM (note, you are changing the amount 
of memory, not the "maximum" amount that could be allocated.
xl mem-set memtest 10g


Now you should see the VM has extra RAM available.

> So, from these test it seems that the take-home lesson is to never use
> "xl mem-max" since it has no effect.
>
> Where am I wrong?
>
> Thank you very much for your help.
>
> Best,
>
> Marco
>
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Regards,
Adam

--
Adam Goryachev Website Managers www.websitemanagers.com.au

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