Date:	Mon, 7 Mar 2011 12:40:01 +0200
From:	Amir Goldstein <amir73il@...il.com>
To:	Lukas Czerner <lczerner@...hat.com>
Cc:	"Ted Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org,
	sandeen@...hat.com
Subject: Re: [PATCH 4/4] e2fsck: Add QCOW2 support

On Tue, Mar 1, 2011 at 1:42 PM, Lukas Czerner <lczerner@...hat.com> wrote:
> 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.

Did you consider the possibility to use QCOW2 format for doing a "tryout"
fsck on the filesystem with the option to rollback?

If QCOW2 image is created with the 'backing_file' option set to the origin
block device (and 'backing_fmt' is set to 'host_device'), then qemu-nbd
will be able to see the exported image metadata as well as the filesystem
data.

You can then do an "intrusive" fsck run on the NBD, mount your filesystem
(from the NBD) and view the results.

If you are satisfied with the results, you can apply the fsck changes to the
origin block device (there is probably a qemu-img command to do that).
If you are unsatisfied with the results, you can simply discard the image
or better yet, revert to a QCOW2 snapshot, which you created just before
running fsck.

Can you provide the performance figures for running fsck over NBD?

>
> 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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