Hi, Indeed, you fill up the last blocks that are not big enough for my defined blocksize. But those blocks are very likely to be empty, also the loss of max 1024k - 1 of datastorage isn't really bothering me. The requirement to fill out the empty space with zeroes and then take down the vm to create a new qcow2 image is what bothers me. I wish to reclaim the empty space from a qcow2 image in a more efficient way, preferably without creating a new qcow2 image. (no downtime would be a nice bonus) Rob 2013/8/20 Tony Su <tonysu@su-networking.com>:

Haven't tried the following to zero a virtual disk, but the following should be fast, make the bs as big as you want (maybe 10x for very large disks). Of course, the bigger you make zero.big.file the longer it <might> take to create zero.file. My code zeroes all bytes in 2 steps whereas yours doesn't zero all.

dd if=/dev/zero of=zero.big.file bs=1024 count=102400 cat /dev/zero > zero.file rm zero.big.file rm zero.file

Tony

On Aug 20, 2013 1:39 AM, "Rob Verduijn" <rob.verduijn@gmail.com> wrote:

...

Hi,

Using separate images for partitions makes the script a tiny bit less complex.

I've already been scripting the exercise, the use off multiple partition images makes it only a tiny bit more complex. qemu-nbd simply adds another device for each additional partition.

#!/binbash modprobe nbd max_part=16 #number off partitions qemu-nbd -c --nocache --aio=native /path/to/image.qcow2 mount /dev/nbd0 /mnt # I didn't partiotion the image, just formated the image as ext4 dd if=/dev/zero of=/mnt/bigfile bs=1024k; sync; rm /mnt/bigfile; rm bigfile; sync; rm /mnt/bigfile; sync sleep 2 umount /dev/nbd0 # umount the device qemu-nbd /dev/nbd0 # clean up nbd devices or they will bite you qemu-img convert -c -O qcow2 /path/to/image.qcow2 /path/to/shrunk.qcow2 mv /path/to/image.qcow2 /path/to/image.qcow2.bak mv /path/to/shrunk.qcow2 /path/to/image.qcow2

The performance of qemu-nbd is rather poor, using writeback cache is not really an option since you always need to wait for the zeroes to be actually written to the hd. Also writeback is very hazardous if you use a script to umount and disconnect the nbd device, image corruption is very likely to happen since sync doesn't apply to nbd0 devices and blockdev --flushbufs /dev/nbd0 isn't foolproof either when scripting. (ctrl-c the script at the wrong time and you are in for a recovery)

So the script isn't that difficult, neither is the use of multiple partitions in an image. The biggest drawback is, downtime, filling the image with zeroes takes even longer offline than online. Ok you could have a cronjob in the vm that does that nightly, but I can imagine issues when the hd of a vm is at filled to the max at regular intervals (once a month ??), even at expected times and only for a short moment. Also that would mean the shrinking cronjob is required on the host (I want the shrinking done as soon as possible after the zeroing) This has to be timed properly with the zero cronjob of the guest, this becomes rather complex with every additional guest.

Regards Rob

2013/8/19 Tony Su <tonysu@su-networking.com>:

...

Not specific to qcow In the past if I wanted to partition files I'd deploy multiple disks instead of partitions. I don't knowi if there a significant overhead diff but I found performance did not suffer. Once on separate disks, should be trivial to script the procedure.

You can execute your conversion on any storage, just as fast as possible. So, for instance can even be cheap temporary attached storage.

And AFAIK has to be done offline although I suppose a fancy live migraation could be implemented so you're not offline. Tony

On Aug 19, 2013 11:50 AM, "Rob Verduijn" <rob.verduijn@gmail.com> wrote:

...

Hello all,

I'm looking for a clever way to shrink qcow2 images.

what I do now is :

1 in the vm delete the files I don't need (tempfiles, spoolfiles, junkfiles, etc, etc) 2 fill the empty space with zeros dd if=/dev/zero of=bigfile bs=1024k ; sync ; rm bigfile ; sync 3 shut down the vm 4 convert the qcow2 to a fresh new qcow2 qemu-img convert -c -O qcow2 orig.qcow2 shrunk.qcow2 mv orig.qcow2 orig.qcow2.bak mv shrunk.qcow2 orig.qcow2 5 test the vm 6 delete the backup if it works

Now this is fine if you got plenty space and just want to tidy up a bit before it turns bad.

But what if there is no space, and adding extra partitions is not that easy. (imagine an offsite, far off location that is a real pain to get into due to serious security hassle)

Or if you are me and are seriously annoyed by the cumbersome exercise above and have a gut feeling that there should be a more elegant way (preferably one without downtime)

The above exercise requires to be done on all the partitions of the qcow2 image if there is more than one, adding to my irritation of the exercise.

Does anybody have any ideas on a more elegant solution ?

cheers Rob -- To unsubscribe, e-mail: opensuse-virtual+unsubscribe@opensuse.org To contact the owner, e-mail: opensuse-virtual+owner@opensuse.org

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