From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nikola Ciprich Subject: Re: consistent LVM snapshot of domUs from dom0 Date: Mon, 24 Dec 2007 16:22:28 +0100 (CET) Message-ID: References: <47639AB8.1060209@ncic.ac.cn> <200712191853.22463.mark.williamson@cl.cam.ac.uk> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Return-path: In-Reply-To: <200712191853.22463.mark.williamson@cl.cam.ac.uk> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Mark Williamson Cc: andy@cs.ubc.ca, nikola.ciprich@linuxbox.cz, xen-devel@lists.xensource.com List-Id: xen-devel@lists.xenproject.org Hi Mark, and sorry for not replying for so long. Parallax looks really promising, and I'll surely keep eye on it... > I've been having a look at this too. Would you like to look into sharing the > work? I'd got as far as plumbing some control plane stuff through xm, Xend, > etc to talk to a stub driver in drivers/xen/fssnap/fssnap.c. I've also > identified where I think we'd need to hook in to get the kernel to freeze its > filesystem. I've not tied it all together yet though. Well, sure I'd like to share, I didn't have much time to play with it yet, I've mostly studied a bit on how the DM stuff works, and I think the simplest, yet working solution could be: $ xm freeze_dev xvda1 myDom - this will tell the domU kernel to call freeze_bdev(...) (see fs/buffer.c) for the given device to put it into consistent state, and stop using it for a while then from dom0 You can safely create a snapshot of lv on top of domU's device is sitting: $ lvcreate -s blabla $ xm unfreeze_dev xvda1 myDom - this will tell the domU kernel to call thaw_bdev(..), which will allow domU to continue using device What do You think about it? > Once this feature is available, it'd be cool to get it plugged into various > other layers of tools - there's lots of potential to do interesting things > with it. e.g. > 1) manual backups > 2) integrate automatically with block device snapshots > 3) enable introspection of a domain filesystem e.g. filesystem contents, free > space, checksum files, measure fragmentation, etc - all these can be > performed on the consistent snapshot whilst the domain itself continues to > run on the primary copy > 4) etc etc etc > > Lots of fun stuff :-) Yup, agree :) Cheers! nik > > Cheers, > Mark > >> cheers >> n. >> >> On Sat, 15 Dec 2007, tgh wrote: >>> hi >>> if we use the ParallaxFS to do the filesystem snapshot, does it have the >>> similar problem ,or not? could you clear me? >>> >>> Thanks in advance >>> >>> Mark Williamson ??: >>>>> I'd like to ask a question regarding LVM snapshots: >>>>> normally if You make a snapshot of mounted LVM partition, DM >>>>> infrastructure handles putting fs into consistent state using VFS >>>>> infrastructure. BUT, does it work even if I make snapshot of LV used by >>>>> XEN? >>>> >>>> If your LVM is running in dom0 and you snapshot the storage used by a >>>> domU then, no, it won't snapshot the domU's filesystem automatically. >>>> >>>>> If not, I guess it should not be that hard to fix, I guess we just need >>>>> to notify domU kernel to call needed VFS function to put fs into >>>>> consistent state prior to creating snapshot. >>>> >>>> Yep. >>>> >>>>> Can somebody comment on it, eventually correct me if I'm wrong? >>>> >>>> You're quite right. I've just had a look at it and it looks quite >>>> feasible to do this. The obvious simple way to implement it would be to >>>> add an xm command that causes a domain to freeze its filesystems into a >>>> consistent state and then return when that is done. >>>> >>>> e.g. >>>> >>>> xm create mydomain >>>> xm fs_freeze mydomain >>>> >>>> xm fs_unfreeze mydomain >>>> >>>> Ideally the backup operation would just take a snapshot e.g. using LVM >>>> or qcow. With a bit more tools / storage integration we could have an >>>> "xm safesnapshot" command that would create a coherent snapshot of the >>>> disk. >>>> >>>> It would be awesome. >>>> >>>> Cheers, >>>> Mark > > > > -- > Dave: Just a question. What use is a unicyle with no seat? And no pedals! > Mark: To answer a question with a question: What use is a skateboard? > Dave: Skateboards have wheels. > Mark: My wheel has a wheel! > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xensource.com > http://lists.xensource.com/xen-devel > > --