suggested domain0 memory size

suggested domain0 memory size

[Jan Kundrát]

Hi,
what is minimal recommended memory reservation for domain0? It won't run 
any services apart from sshd and xend and will provide files from /home 
(ext3) to other domains as their block devices. Machine is 466MHz 
Celeron with 256MB of RAM, UDMA/33 IDE interface, and will run about 5 
other domains, so less is more ;-).

TIA,
j.



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Re: suggested domain0 memory size

[Ian Pratt]

> Hi,
> what is minimal recommended memory reservation for domain0? It won't run 
> any services apart from sshd and xend and will provide files from /home 
> (ext3) to other domains as their block devices. Machine is 466MHz 
> Celeron with 256MB of RAM, UDMA/33 IDE interface, and will run about 5 
> other domains, so less is more ;-).

Boot it with 64MB and then use the balloon driver to crank it
down a bit. Give it a swap file to be on the safe side.

Ian


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Re: suggested domain0 memory size

[Jan Kundrát]

>>Hi,
>>what is minimal recommended memory reservation for domain0? It won't run 
>>any services apart from sshd and xend and will provide files from /home 
>>(ext3) to other domains as their block devices. Machine is 466MHz 
>>Celeron with 256MB of RAM, UDMA/33 IDE interface, and will run about 5 
>>other domains, so less is more ;-).
> 
> 
> Boot it with 64MB and then use the balloon driver to crank it
> down a bit. Give it a swap file to be on the safe side.

Thanks, but what do you think about 32MB?

j.



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Re: suggested domain0 memory size

[Jérôme Petazzoni]

>>> what is minimal recommended memory reservation for domain0? It won't 

> Thanks, but what do you think about 32MB?

I had serious problems with low-memory domain0. It caused problems with 
the backend of the block devices of other domains (those domains got I/O 
errors). I would not go below 64 MB, unless maybe by tweaking VM "will 
to swap" to be sure to have enough kernel memory available at anytime.


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Re: suggested domain0 memory size

[Jan Kundrát]

Jérôme Petazzoni wrote:
>> Thanks, but what do you think about 32MB?
> 
> 
> I had serious problems with low-memory domain0. It caused problems with 
> the backend of the block devices of other domains (those domains got I/O 
> errors). I would not go below 64 MB, unless maybe by tweaking VM "will 
> to swap" to be sure to have enough kernel memory available at anytime.

Thanks for feedback. I'm using file-backed VBDs (ie. domains are using 
files stored under domain0's filesystem). BTW, what is the 
lowest-memory-hungry filesystem for this usage? Or should be something 
like LVM better?

TIA,
jkt

-- 
cd /local/pub && more beer > /dev/mouth



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Re: suggested domain0 memory size

[Jan Kundrát]

>>Hi,
>>what is minimal recommended memory reservation for domain0? It won't run 
>>any services apart from sshd and xend and will provide files from /home 
>>(ext3) to other domains as their block devices. Machine is 466MHz 
>>Celeron with 256MB of RAM, UDMA/33 IDE interface, and will run about 5 
>>other domains, so less is more ;-).
> 
> 
> Boot it with 64MB and then use the balloon driver to crank it
> down a bit. Give it a swap file to be on the safe side.

Just a small question - if I use `xm balloon Domain-0 64`, `xm info` 
reports correct amount of memory as free, but `free` says that domain0 
has still $original_memory_size total, but used memory grows up, is it 
correct?

(total, used, free, shared, buffers, cached)
# free | grep Mem
Mem:        116692      29056      87636          0        460       9600
# xm info | grep memory
memory                 : 255
free_memory            : 126
# xm balloon Domain-0 64
# free | grep Mem
Mem:        116692      83536      33156          0        460       9600
# xm info | grep memory
memory                 : 255
free_memory            : 179

TIA,
j.



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Re: suggested domain0 memory size

[Keir Fraser]

> >>Hi,
> >>what is minimal recommended memory reservation for domain0? It won't run 
> >>any services apart from sshd and xend and will provide files from /home 
> >>(ext3) to other domains as their block devices. Machine is 466MHz 
> >>Celeron with 256MB of RAM, UDMA/33 IDE interface, and will run about 5 
> >>other domains, so less is more ;-).
> > 
> > 
> > Boot it with 64MB and then use the balloon driver to crank it
> > down a bit. Give it a swap file to be on the safe side.
> 
> Just a small question - if I use `xm balloon Domain-0 64`, `xm info` 
> reports correct amount of memory as free, but `free` says that domain0 
> has still $original_memory_size total, but used memory grows up, is it 
> correct?

>From the point of view of the kernel, ballooned memory is actually "in
use". This is because the balloon is inflated by requesting memory
from the kernel allocator -- the kernel assumes this memory is in use
by the balloon driver, when in fact it is returned to Xen.

So, 'free' and 'cat /proc/meminfo' will both mislead you. You need to
subtract the balloon size from the 'total memory' field to find out
how much memory is *really* in use by the domain.

 -- Keir

> (total, used, free, shared, buffers, cached)
> # free | grep Mem
> Mem:        116692      29056      87636          0        460       9600
> # xm info | grep memory
> memory                 : 255
> free_memory            : 126
> # xm balloon Domain-0 64
> # free | grep Mem
> Mem:        116692      83536      33156          0        460       9600
> # xm info | grep memory
> memory                 : 255
> free_memory            : 179
> 
> TIA,
> j.
> 
> 
> 
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> Read honest & candid reviews on hundreds of IT Products from real users.
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> Xen-devel@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/xen-devel



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Re: suggested domain0 memory size

[Jan Kundrát]

> From the point of view of the kernel, ballooned memory is actually "in
> use". This is because the balloon is inflated by requesting memory
> from the kernel allocator -- the kernel assumes this memory is in use
> by the balloon driver, when in fact it is returned to Xen.

Thanks. Is this true also for other domains than dom0?

j.



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Re: suggested domain0 memory size

[Keir Fraser]

> > From the point of view of the kernel, ballooned memory is actually "in
> > use". This is because the balloon is inflated by requesting memory
> > from the kernel allocator -- the kernel assumes this memory is in use
> > by the balloon driver, when in fact it is returned to Xen.
> 
> Thanks. Is this true also for other domains than dom0?
> 
> j.
> 

This is true in any context you use a balloon driver. Dom0 or DomU.

 -- Keir


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