From: Lukas Czerner Subject: Re: [PATCH 4/4] e2fsck: Add QCOW2 support Date: Mon, 7 Mar 2011 13:40:32 +0100 (CET) Message-ID: References: <1298638173-25050-1-git-send-email-lczerner@redhat.com> <1298638173-25050-4-git-send-email-lczerner@redhat.com> <20110226164442.GD2924@thunk.org> Mime-Version: 1.0 Content-Type: MULTIPART/MIXED; BOUNDARY="8323328-1671297533-1299501635=:5084" Cc: Lukas Czerner , "Ted Ts'o" , linux-ext4@vger.kernel.org, sandeen@redhat.com To: Amir Goldstein Return-path: Received: from mx1.redhat.com ([209.132.183.28]:41760 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750927Ab1CGMki (ORCPT ); Mon, 7 Mar 2011 07:40:38 -0500 In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --8323328-1671297533-1299501635=:5084 Content-Type: TEXT/PLAIN; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT On Mon, 7 Mar 2011, Amir Goldstein wrote: > On Tue, Mar 1, 2011 at 1:42 PM, Lukas Czerner 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 > >> > .raw.XXXXXX, where X chosen randomly. > >> > > >> > Signed-off-by: Lukas Czerner > >> > >> 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. But this is something you can do even now. You can mount the qcow2 metadata image without any problems, you just will not see any data. But I can take a look at this functionality, it seems simple enough. > > Can you provide the performance figures for running fsck over NBD? Well, unfortunately I do not have access to the same machine anymore, but I have simple results which has been done elsewhere, but due to lack of proper storage this has been done on loop device (should not affect raw and qcow2 results). [+] fsck raw image real 0m30.176s user 0m22.397s sys 0m2.289s [+] fsck NBD exported qcow2 image real 0m31.667s user 0m21.561s sys 0m3.293s So you can see that performance here is a bit worse (5%). Thanks! -Lukas > > > > > 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 > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > > the body of a message to majordomo@vger.kernel.org > > More majordomo info at ?http://vger.kernel.org/majordomo-info.html > > > -- --8323328-1671297533-1299501635=:5084--