Date:	Mon, 16 May 2011 15:01:40 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Lukas Czerner <lczerner@...hat.com>
CC:	linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH 0/3 v2] Add qcow2 support

On 5/16/11 10:43 AM, Lukas Czerner wrote:
> Hello,
> 
> it has been a while since I have posted this last time. Noone seems to care
> very much, but I think this is really useful feature for e2image so I give
> it a try once more.

I care!  I'm currently bunzip2'ing a multi-terabyte image.

Zipped, it is 14M.  After about 6 hours, it has reached an offset
of 3T, and it has written ...

... 131 Megabytes.

In 6 hours.

If ext4 dreams of actual large filesystem support, being able to handle
metadata dumps at this scale will be critical.

So I should review this ;)

Thanks,
-Eric

> These patches adds a qcow2 support to e2image. We all know how painful! is
> using e2image on huge filesystems, especially when you need to transfer 
> the images. Qcow2 images are different, due to its format it is not sparse
> yet, it is space efficient because it packs all used file system metadata
> close together, hence avoiding sparseness. So it is really easy to generate,
> trasfer and convert qcow2 images.
> 
> Now in order to check the image, or use debugfs you have to convert it into
> the qcow2 image. Unlike targzipping it is *A LOT* faster, however you still
> have to have the space for the raw image, but I do not see that as a big
> problem and hey it is a HUGE improvement from what we have today! However
> it would be possible to create new iomanager which does understand qcow2
> format directly, but I am not going to do that, since there is no library
> and it would mean copy/change and maintain part of qemu code and I do not
> think this is worth it. But anyone interested in this, feel free to look
> at it.
> 
> Also, remember that if you really do not want to convert the image
> because of file size limit, or whatever, you can always use qemu-nbd to
> attach qcow2 image into nbd block device and use that as regular device.
> 
> 
> [ Note that the last patch (#3) might not make it into the list, because
> it contains new tests and file system images and it is 1.2MB big. If you
> would like to see it please let me know and I will send it to you directly ]
> 
> Now here are some numbers:
> --------------------------
> 
> I have 6TB raid composed of four drives and I flooded it with lots and
> lots of files (copying /usr/share over and over again) and even created
> some big files (1M, 20M, 1G, 10G) so the number of used inodes on the
> filesystem is 10928139. I am using e2fsck form top of the master branch.
> 
> Before each step I run:
> sync; echo 3 > /proc/sys/vm/drop_caches
> 
> exporting raw image:
> time .//misc/e2image -r /dev/mapper/vg_raid-lv_stripe image.raw
> 
>         real    12m3.798s
>         user    2m53.116s
>         sys     3m38.430s
> 
>         6,0G    image.raw
> 
> exporting qcow2 image
> time .//misc/e2image -Q /dev/mapper/vg_raid-lv_stripe image.qcow2
> e2image 1.41.14 (22-Dec-2010)
> 
>         real    11m55.574s
>         user    2m50.521s
>         sys     3m41.515s
> 
>         6,1G    image.qcow2
> 
> So we can see that the running time is essentially the same, so there is
> no crazy overhead in creating qcow2 image. Note that qcow2 image is
> slightly bigger because of all the qcow2 related metadata and it's size
> really depends on the size of the device. Also I tried to see how long
> does it take to export bzipped2 raw image, but it is running almost one
> day now, so it is not even comparable.
> 
> e2fsck on the device:
> time .//e2fsck/e2fsck -fn /dev/mapper/vg_raid-lv_stripe
> 
>         real    3m9.400s
>         user    0m47.558s
>         sys     0m15.098s
> 
> e2fsck on the raw image:
> time .//e2fsck/e2fsck -fn image.raw
> 
>         real    2m36.767s
>         user    0m47.613s
>         sys     0m8.403s
> 
> We can see that e2fsck on the raw image is a bit faster, but that is
> obvious since the drive does not have to seek so much (right?).
> 
> Now converting qcow2 image into raw image:
> time .//misc/e2image -r image.qcow2 image.qcow2.raw
> 
>         real    1m23.486s
>         user    0m0.704s
>         sys     0m22.574s
> 
> It is hard to say if it is "quite fast" or not. But I would say it is
> not terribly slow either. Just out of curiosity, I have tried to convert
> raw->qcow2 with qemu-img convert tool:
> 
> time qemu-img convert -O raw image.qcow2 image.qemu.raw
> ..it is running almost an hour now, so it is not comparable as well :)
> 
> 
> 
> Please review!
> 
> Thanks!
> -Lukas
> 
> 

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