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From: | Uri Lublin |
Subject: | Re: [Qemu-devel] Re: Problems KVM-84 |
Date: | Thu, 12 Mar 2009 00:59:52 +0200 |
User-agent: | Thunderbird 2.0.0.19 (X11/20090105) |
Jamie Lokier wrote:
You assume there is a file system installed. If I want to use e.g. LVM's logical volume, I can not tell the size of the qcow2 image. I want to be able to set a watermark and handle out-of-disk-space before it actually happens.Anthony Liguori wrote:Any quick ideas? Seems like this is broken by design. Unless we can find a quick fix, I'm going to revert this in the stable tree.block-qcow2: keep highest allocated byte (Uri Lublin) We want to know the highest written offset for qcow2 images. This gives a pretty good (and easy to calculate) estimation to how much more allocation can be done for the block device. It can be usefull for allocating more diskspace for that image (if possible, e.g. lvm) before we run out-of-disk-space Signed-off-by: Uri Lublin <address@hidden> Signed-off-by: Anthony Liguori <address@hidden> Scanning the file at startup is slow. We need to find a better way.I still don't understand the point in that feature. You have the size of the qcow2 file (how much data is used), and the size of the image it's representing (how much it could expand to). What does the highest written offset help you anticipate?
Many guest filesystems write data all over the block device, not concentrated at the start. (E.g. ext2/3/4 and it's block groups).
qcow2 allocates disk space regardless of the guest write offset.And if it happen that the image is sparse (or fragmented), num_free_bytes can tell us that.
Regards, Uri.
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