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Re: [Xen-users] memory quesion

To: "Fajar A. Nugraha" <fajar@xxxxxxxxx>
Subject: Re: [Xen-users] memory quesion
From: Paras pradhan <pradhanparas@xxxxxxxxx>
Date: Fri, 11 Dec 2009 16:57:21 -0600
Cc: Xen Users <xen-users@xxxxxxxxxxxxxxxxxxx>
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On Fri, Dec 11, 2009 at 4:16 PM, Fajar A. Nugraha <fajar@xxxxxxxxx> wrote:
On Sat, Dec 12, 2009 at 3:28 AM, Paras pradhan <pradhanparas@xxxxxxxxx> wrote:
>>> My system has 64GB of Physical RAM. When I boot the system with Xen
>>> kernel,
>>> I can see only 32GB assigned to dom0. my xm info o/p is;
>>> total_memory           : 65530
>>> free_memory            : 31513
>>> My understanding is when the system is booted with xen kernel, all of the
>>> memory should be assigned to dom0.

No, it shouldn't. In fact, it's actually best practice to LIMIT the
ammount of dom0 memory from the start.

>>> Os is: RH 5.4 64bit.
>> This is a limitation of the Xen kernel; only 32GB will be visible to Dom0.
>> The "missing" 32GB can still be allocated to DomU's, however.

AFAIK it's a "limitation" of RHEL's kernel-xen. They did it for a very
good reason though: providing a usable dom0 system on servers with >=
32GB memory.

For a dom0 to be usable and stable, i guess 32 GB of memory is not needed. Around 1GB should be enough(?) and this 1 GB is set using dom0_mem=1Gb param? correct me if i am wrong.


You could probably override that behavior with some kernel command
line options, but it's not recommended.

> Do I need mem=64G in grub or not ?

If you plan to have some domUs running on it, you can leave it as it
is. domU will be able to use the 32GB currently not in use by dom0.

In fact, domUs can also use some memory currently used by dom0, so if
already have an idea how much memory domUs will be using in total, you
can (later) limit dom0 memory usage to start with less memory (I
usually set it to somewhere between 500M - 1G).

So is it still true that domU can use memory more than 32GB whereas it is showing up only 32G? I will having around 20 virtual machines and each will use 1 to 2 CPU and 2 GB of RAM.

So,  i need some suggestions here.

Thanks !



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