From: Lukas Czerner Subject: Re: [PATCH v1 00/30] Ext4 snapshots Date: Thu, 9 Jun 2011 12:17:23 +0200 (CEST) Message-ID: References: <1307459283-22130-1-git-send-email-amir73il@users.sourceforge.net> Mime-Version: 1.0 Content-Type: MULTIPART/MIXED; BOUNDARY="8323328-1187090254-1307614648=:4138" Cc: david@lang.hm, Lukas Czerner , Yongqiang Yang , linux-ext4@vger.kernel.org, tytso@mit.edu, linux-kernel@vger.kernel.org, sandeen@redhat.com, Arvin Schnell To: "Amir G." Return-path: Received: from mx1.redhat.com ([209.132.183.28]:43407 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752447Ab1FIKRl (ORCPT ); Thu, 9 Jun 2011 06:17:41 -0400 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. --8323328-1187090254-1307614648=:4138 Content-Type: TEXT/PLAIN; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT On Thu, 9 Jun 2011, Amir G. wrote: > On Thu, Jun 9, 2011 at 11:13 AM, wrote: > > On Thu, 9 Jun 2011, Amir G. wrote: > > > >> On Thu, Jun 9, 2011 at 9:50 AM, Lukas Czerner wrote: > >>> > >>> On Thu, 9 Jun 2011, Yongqiang Yang wrote: > >>> > >>>> On Thu, Jun 9, 2011 at 11:18 AM, Amir G. > >>>> wrote: > >>>>> > >>>>> On Thu, Jun 9, 2011 at 4:59 AM, Yongqiang Yang > >>>>> wrote: > >>> > >>> You see, when it comes to the full fs snapshots I am not convinced that > >>> it is *very* useful, yes it might have some users, but you can alway > >>> take the safe way and do lvm snapshots (or better use the new multisnap) > >>> for backup, without need to modify stable filesystem code. > >>> > >> > >> You think like a developer. Try talking to some sys admins. > >> Especially ones that worked with Solaris/ZFS or NetApp. > >> See what they think about snapshots and about the LVM alternative... > >> Snapshots have addictive qualities. Ones you've used them, you can't > >> go back to not having them. > >> Imagine how people used to live before the 'Undo' button and imagine > >> that your employer forced you to use an editor without an Undo button. > >> This is the kind of feedback I got from sys admins that moved from Solaris > >> to Linux. > > > > as a sysadmin, it's a _wonderful_ tool to have for any system that has > > people editing/saving files on directly. > > Thank you david. Finally some positive feedback from the people > for whom the feature is intended for :-) No one is arguing about the advantages of snapshots. I think that we are all clear on this. Snapshots are useful. > > > > >> > >>> Also, I do not buy the whole argument of "not have to create separate > >>> disk > >>> space for snapshot". It is actually better for sysadmins, because you > >>> have perfect control on what is going on, how much space is used for > >>> your snapshots and how much is used by your data. You can always easily > >>> extend the snapshot volume, or let it die silently when it is too old > >>> and too big. > >>> > >> > >> Seriously, Lukas, talk to sys admins. > >> Letting the snapshot die silently is the worst possible thing that a > >> snapshots > >> implementation can do (for long lived snapshots). > > > > that depends on the site policy. > > > > sometimes it is better to loose snapshots than to run out of disk space and > > halt the system, sometimes you would rather halt the system. > > > > the policy of what happens when you run out of space should not be a kernel > > decision, the desired behavior varies far too much. > > > > this includes being able to say things like "I want to always have 10% of my > > disk allocated to snapshots, but if there's more free space, go ahead and > > use it, but always keep at least 10% of the disk free so that you don't have > > to halt new writes while you clear space" > > > > or > > > > "if you run out of space, try and keep the oldest snapshot and the newest > > snapshot, delete other snapshots in between before touching either of these" > > > > I fully agree. > AFAIK, there is no user space tool to manage snapshots to this level for Linux. > The only snapshot manager I know about is snapper: > http://en.opensuse.org/Portal:Snapper, which we are working on adding > ext4 snapshots support to. > Snapper does not have the free space based policy to the best of my knowledge, > but it could be improved to monitor free disk space. > > A tool like that does not need any further kernel changes from > ext4 and btrfs to implement the policies suggested above. > However, with LVM snapshots, some of these policies (like use whatever space you > have free in the filesystem) are simply not possible. And why is that ? With LVM you can shrink or extent volumes at will, I do not think this is a problem at all, moreover, you can always add more drives to resize your existing volumes to. > > > >>> How does it actually work on ext4 snapshots ? When you're going to > >>> rewrite a file, you will never know how much disk space it'll take in > >>> advance, am I right ? Is the filesystem accounting for the snapshot size > >>> as well ? or is it hidden ? > >> > >> It's not hidden, it's accounted for as a regular file (usually owned by > >> root). > >> You need a bit of scripting to gather the disk space used by snapshots > >> (du). > > > > the worst case when you re-write a file is that it will take the full amount > > of space that the file currently takes (as if you wrote a new copy of the > > file and some process had a filehandle open on the old copy, preventing the > > space from being reclaimed, so it's far from being a new problem) > > No. it's a new problem. > When you have a large db, which does random writes to an exiting db file, > it does not expect ENOSPC, when updating an existing record or index. > Only by keeping enough free disk space in the system at all times, can you > avoid this kind of problems. You can very well avoid this kind of problems when you separate filesystem and snapshots, that is what LVM can do easily. > > > > > see the note above about the need to be able to remove snapshots when you > > are out of space. > > > > since snapshots tend to be small compared to the filesystems they protect > > (not in all cases, but if you are covering the entire system with one > > snapshot that would be the way to bet), having the ability to put the > > snapshot metadata off on a smaller/faster disk would be helpful. Very easy to do with dm-multisnap. > > Helpful for which workload? > For reading from snapshots? Yes, that would be faster. > For writing to the filesystem? I demonstrated that the performance > overhead is near zero. > > > > > having the ability to snapshot just specific files/directories would be a > > killer feature IMHO > > I agree to that, but I don't think the ext4 will be able to provide > that to the full extent. And that is for being fs level snapshots a huge drawback. > > > > > David Lang > > -- > > 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 > > > --8323328-1187090254-1307614648=:4138--