Re: [Qemu-devel] The image size of instance VM keeps growing

[Charles . Tsai-蔡清海-研究發展部]

Stefan,

        I made two test cases here.

        1) case one:
                A 32-bit Windows 7 VM with 8G virtual disk size.
                In this test, the virtual disk is almost full.
                Like what you said, the instance VM size only increased 2% 
initially and only a little when virtual disk size is almost                    
     full.

        2) case two: (we think it is a bug)

                A 64-bit Windows 7 VM with 64G virtual disk size.
                Base image size: 19G

                Here is what I found

                The increase size of instance VM is significantly different 
from the decrease size of virtual disk.
                The decrease size of virtual disk stands for the new data 
written into the disk by the OS.

                Here are the readings I collected at differ time slot.

                1)
                Instance VM size: 1.6G
                Virtual disk free space: 45792534528 (read from guest OS)

                2)
                Instance VM size: 2.1 G
                Virtual disk free space: 45786300416 (read from guest OS)       

                You can find that the instance VM increase 0.5G but virtual 
disk space only reduces around 6MB.
                This process will continue as long as the VM is running.

                What do you think about this? Let me know if you need more 
explanations from me.



-----Original Message-----
From: Charles.Tsai-蔡清海-研究發展部 
Sent: Thursday, May 24, 2012 6:41 PM
To: 'Stefan Hajnoczi'
Cc: address@hidden; Jonah.Wu-吳君勉-研究發展部
Subject: RE: [Qemu-devel] The image size of instance VM keeps growing

Stefan,

        Can you explain why a 64KB could be allocated for each sector write?

        Right now, I am testing a VM with 8G virtual disk space.
        The disk only has 12.6 MB free space left when I run the test.
        I am going to see what will happen in this test case.



-----Original Message-----
From: Stefan Hajnoczi [mailto:address@hidden 
Sent: Wednesday, May 23, 2012 8:57 PM
To: Charles.Tsai-蔡清海-研究發展部
Cc: address@hidden; Jonah.Wu-吳君勉-研究發展部
Subject: Re: [Qemu-devel] The image size of instance VM keeps growing

On Wed, May 23, 2012 at 11:47 AM, Charles.Tsai-蔡清海-研究發展部
<address@hidden> wrote:
>        After the VM ran for overnight, the image size for the running VM grew 
> up to 3G bytes.

What is the size of the backing file?  3G is small for a Windows guest.  Any 
sector that is written to could cause 64 KB to be allocated in the image file.

If qemu-img check sees no inconsistencies then it seems the guest is writing to 
previously untouched regions of the disk...causing the
qcow2 to grow.

However, the qcow2 file should not grow much beyond its virtual disk size.  So 
if you find the qcow2 is 3G but the virtual disk size is 1G, then there is a 
bug.  But from what you've posted so far I suspect the guest is simply writing 
to the disk.

Stefan