Date:	Tue, 1 Mar 2011 12:42:04 +0100 (CET)
From:	Lukas Czerner <lczerner@...hat.com>
To:	"Ted Ts'o" <tytso@....edu>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org,
	sandeen@...hat.com
Subject: Re: [PATCH 4/4] e2fsck: Add QCOW2 support

On Sat, 26 Feb 2011, Ted Ts'o wrote:

> On Fri, Feb 25, 2011 at 01:49:33PM +0100, Lukas Czerner wrote:
> > This commit adds QCOW2 support for e2fsck. In order to avoid creating
> > real QCOW2 image support, which would require creating a lot of code, we
> > simply bypass the problem by converting the QCOW2 image into raw image
> > and than let e2fsck work with raw image. Conversion itself can be quite
> > fast, so it should not be a serious slowdown.
> > 
> > Add '-Q' option to specify path for the raw image. It not specified the
> > raw image will be saved in /tmp direcotry in format
> > <qcow2_filename>.raw.XXXXXX, where X chosen randomly.
> > 
> > Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> 
> If we're just going to convert the qcow2 image into a raw image, that
> means that if someone sends us a N gigabyte QCOW2 image, it will lots
> of time (I'm not sure I agree with the "quite fast part"), and consume
> an extra N gigabytes of free space to create the raw image.
> 
> In that case, I'm not so sure we really want to have a -Q option to
> e2fsck.  We might be better off simply forcing the use of e2image to
> convert the image back.
> 
> Note that the other reason why it's a lot better to be able to allow
> e2fsck to be able to work on the raw image directly is that if a
> customer sends a qcow2's metadata-only image from their 3TB raid
> array, we won't be able to expand that to a raw image because of
> ext2/3/4's 2TB maximum file size limit.  The qcow2 image might be only
> a few hundreds of megabytes, so being able to have e2fsck operate on
> that image directly would be a huge win. 
> 
> Adding iomanager support would also allow debugfs to access the qcow2
> image directly --- also a win.
> 
> Whether or not we add the io_manager support right away (eventually I
> think it's a must have feature), I don't think having a "decompress a
> qcow2 image to a sparse raw image" makes sense as an explicit e2fsck
> option.  It just clutters up the e2fsck option space, and people might
> be confused because now e2fsck could break because there wasn't enough
> free space to decompress the raw image.  Also, e2fsck doesn't delete
> the /tmp file afterwards, which is bad --- but if it takes a large
> amount of time to create the raw image, deleting afterwards is a bit
> of waste as well.  Probably better to force the user to manage the
> converted raw file system image.
> 
> 					- Ted
> 

Hi Ted,

sorry for late answer, but I was running some benchmarks to have some
numbers to throw at you :). Now let's see how "qite fast" it actually is
in comparison:

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 :)

e2fsck on the qcow2 image.
time .//e2fsck/e2fsck -fn -Q ./image.qcow2.img.tmp image.qcow2

	real    2m47.256s
	user    0m41.646s
	sys     0m28.618s

Now that is surprising. Well, not so much actually.. We can see that
e2fsck check on the qcow2 image, including qcow2->raw conversion is a
bit slower than checking raw image (by 7% which is not much) but it is
still faster than checking device itself. Now, the reason is probably
that the raw image we are creating is partially loaded into memory, hence
accelerate e2fsck. So I do not think that converting image before check
is such a bad idea (especially when you have enough memory:)).

I completely agree that having io_manager for the qcow2 format would be
cool, if someone is willing to do that, but I am not convinced that it
is worth it. Your concerns are all valid and I agree, however I do not
think e2image is used by regular unexperienced users, so it should not
confuse them, but that is just stupid assumption :).

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.

Regarding the e2fsck and the qcow2 support (or -Q option), I think it is
useful, but I do not really insist on keeping it and as you said we can
always force user to use e2image for conversion. It is just, this way it
seems easier to do it automatically. Maybe we can ask user whether he
wants to keep the raw image after the check or not ?

Regaring separate qcow2.h file and "qcow2_" prefix. I have done this
because I am using this code from e2image and e2fsck so it seemed
convenient to have it in separate header, however I guess I can move it
into e2image.c and e2image.h if you want.

So what do you think.

Thanks!
-Lukas
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