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Re: [Xen-devel] Logging Access to HDD

To: Sebastian Biedermann <biedermann@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Logging Access to HDD
From: Michal Novotny <minovotn@xxxxxxxxxx>
Date: Tue, 19 Apr 2011 11:54:30 +0200
Cc: James Harper <james.harper@xxxxxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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On 04/19/2011 11:44 AM, Sebastian Biedermann wrote:
> Am 19.04.2011 11:35, schrieb James Harper:
>>> Dear List,
>>> I´m working in research and I tought this list could be
>>> a good adress for my questions.
>>> I want to log the disk accesses of the virtual hvm instances running in Xen.
>>> That means for the start I want to log the write querys of a running
>>> domU instance in the dom0 instance.
>>> So I´m trying to modify the Xen 3.2.1 source code,
>>> but actually I was not able to find a good entry point to do this.
>>> For now, I want to log the disk accesses of a running windows 7 domU
>>> instance.
>>> The best what could happenis that I could see even the source and target
>>> of a hdd write query.
>>> Does anyone have an idea how I could do this on a good way?
>>> Which Xen source file / function should I modify? Where is the best
>>> entry point to do this?
>> HVM access can either be emulated PCI IDE, or PV. For the emulated access 
>> you would hook into qemu, I think. For PV access you would need to hook into 
>> whatever block device backend you are using.
>> Do you just want to count reads and writes, or do you want to log every 
>> single byte read/written?
>> James
> I dont need to log every single byte, it would be enough to know which 
> file is accessed by the domU inside its image.
> So when I use HVM I need to modify qemu and not the xen source?
> thanks
Sebastian, QEMU is used by Xen for HVM guests. It's in the Xen source
codes in the tools/ioemu-dir so look there and here it depends on disk
type you're having - whether SCSI or IDE disk or whether you're using PV
drivers. This can be found in the Xen domain configuration, i.e. if
you're using file:/path/to/image,xvda then you're using PV drivers (they
have to be installed in the guest), for hda instead of xvda you're using
IDE disk and for sda instead of xvda you're using SCSI disk. If you're
using PV drivers then you have to modify the PV drivers themselves and
not QEMU so I'd recommend to use SCSI or IDE disk instead. For SCSI disk
it's the best since you have the logic in the ioemu-dir/hw/scsi-disk.c


Michal Novotny <minovotn@xxxxxxxxxx>, RHCE
Virtualization Team (xen userspace), Red Hat

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