From mboxrd@z Thu Jan 1 00:00:00 1970 From: Zdenek Kabelac Subject: Re: snapshot-origin freezes system - what am I doing wrong? Date: Fri, 15 May 2015 20:56:15 +0200 Message-ID: <5556414F.5030102@redhat.com> References: <5555CE5D.3030404@web2web.at> <5555E25A.2040106@redhat.com> <55562315.4060707@web2web.at> <555633DE.9060002@redhat.com> <55563F98.3070809@web2web.at> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <55563F98.3070809@web2web.at> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: dm-devel@redhat.com List-Id: dm-devel.ids Dne 15.5.2015 v 20:48 Atom2 napsal(a): > Am 15.05.15 um 19:58 schrieb Zdenek Kabelac: >> Dne 15.5.2015 v 18:47 Atom2 napsal(a): >>> Am 15.05.15 um 14:11 schrieb Zdenek Kabelac: >>>> Dne 15.5.2015 v 12:45 Atom2 napsal(a): >>>>> Hello list, >>>>> I am trying to setup a cow snapshot for a LV that is used as a master image >>>>> for a number of VMs. The idea basically is to be able to update the master >>>>> image even when VMs are up and running; the VMs should then still see the >>>>> old >>>>> state of the image and only when they are restarted they should connect >>>>> to the >>>>> new image. >>>>> Searching the net seemed to point towards a snapshot-origin/snapshot >>>>> solution >>>>> - however I am unable to get this to work. Information on the net seems >>>>> to be >>>>> sparse, so I though I'd ask the experts on the list. Here are my steps: >>>>> >>>>> 1.) I have a LV in volume group VG named master.ROOT >>>>> (/dev/mapper/VG-master.ROOT), 8GB, formatted as ext4 >>>>> 2.) I create a sparese file: truncate -size=8G /tmp/snapshot >>>>> 3.) losetup -f /tmp/snapshot --> gives /dev/loop0 >>>>> 4.) dmsetup create mytest.img --table "0 $(blockdev --getsz >>>>> /dev/mapper/VG-master.ROOT) snapshot-origin /dev/mapper/VG-master.ROOT >>>>> 5.) dmsetup create mytest.img.cow --table "0 $(blockdev --getsz /dev/loop0) >>>>> snapshot /dev/mapper/VG-master.ROOT /dev/loop0 P 8" >>>>> >>>>> So far so good ... however, when I try to mount the origin device by >>>>> 6.) mount /dev/mapper/mytest.img >>>>> the mount call doesn't return and the system gets unresponsive/freezes up >>>>> to a >>>>> point when OOM-killer is being invoked. Login attempts on the console >>>>> time out >>>>> and in essence it is only possible to reboot the system using >>>>> magic-sysreq key >>>>> combinations. >>>>> >>>>> I'd be very much obliged if someone in the know could provide me with >>>>> information what's wrong with this approach. >>>>> >>>>> Many thanks in advance Atom2 >>>> >>>> Do you have any scientific reason to not use LVM2 here ? >>> As far as I understand the snapshot-origin target with a cow snapshot is not >>> (yet) directly supported by LVM2. >> >> Just curious - from where have you got this idea ?? > Well I simply haven't found anything within LVM2 that supports my use case. > Everything I have found is about buffering writes to the original device such > that the original device appears unchanged. > I am, however, curious as well: What makes you think that my use case is > covered by pure LVM2? Do you have a link with the steps required - that would > be great! >> >>> >>> Therefore I have so far worked from an assumption that I need to use dmsetup >>> directly to solve my use case. If, however, you tell me otherwise and are able >>> to show me how to use LVM2 instead, I am more than happy to go down that >>> route. Let me once again outline my use case: >>> >>> I have a LV (that's the master image/template which was actually setup with >>> LVM2) which is maintained from within a template VM and serves as the (r/o) >>> root image for a number of other (dependent) virtual machines. All dependent >>> virtual machines mount that template LV r/o and overlay it with an overlayfs >>> as a r/w layer for write access. >>> I now want to be able to update the master image from within the template VM >>> and be able to update it at any time; running dependent VMs should however not >>> see any changes to the image while they are up, hence the requirement to have >>> a snapshot-origin target and a snapshot that would cow buffer any changes in >>> the template (origin) target until such time the dependent VM is restarted. At >>> that point in time the dependent VM would (if required setup and) connect to a >>> new snapshot. >>> >>> I hope that clarifies my setup. >>> >>> Furthermore, IMHO and in any case mounting a dm-target should under no >>> circumstances in essence bring down a machine - even if something is horribly >>> wrong. >>> >>>> Management of snapshot target is not trivial - especially the order >>>> of individual table loads and resumes. >>>> >>>> You could look at 'lvcreate -s -vvvv' if you are interested in ioctl >>>> ordering of all operations here. >>>> >>> I am probably missing something here, but my steps listed above do not involve >>> lvcreate ... so how could I use that -vvv? >>> >> >> Well even kernel documentation for the snapshot target itself is rather >> referencing usage of snapshot via lvm2 - and it's for a reason. > I have seen that, but I got the impression, that's the other way round to what > I require: It creates a snapshot that copies writes to the original device in > order for the original volume to be in a consitent state e.g. for bacups; > those writes will then later be merged back to the original volume. That's, > however, unfortunately not what I require. What I require is that upon a write > to the original device the _old_ block is copied to the snapshot cow device so > that any process accessing the snapshot has a consistent (and unchanged) view > of the original device. The original device itself will change! >> >> https://www.kernel.org/doc/Documentation/device-mapper/snapshot.txt >> >> So if you could use lvm2 - stay with lvm2 and avoid your home-brew dmsetup >> commands - it needs deep understanding how the old snapshot target works: >> >> http://people.redhat.com/agk/talks/LVM2-LinuxTag2006/ > It looks as if I am not smart enough and (at least) in the area of > device-manager (which is expected) you are much smarter than me. So let me > pick your brain: Would you be so kind to outline the steps required for me to > be able to setup my use case with LVM2: Again, I need a snapshot device that > only stores changed blocks from the snapshot-origin device (which is used > somewhere else) and routes unchanged read requests back to the origin device. > I am still not sure whether we are on the same page here. > > I am also very curious to understand why the exact same steps outlined further > above _do_ work when both the snapshot-origin device and the snapshot device > are based on losetup (spares-)file based block devices and fail when the > snapshot-origin device is a block device created by LVM2 and only the snapshot > device is a losetup (sparse-)file based block device. > > Many thanks again for your patience, but I'd very much appreciate if you could > be more explicit in your (at least for me) rather vague answers. I am also > happy to join a session on IRC - but that channel seems to be pretty orphand > at all times. I'm not saying lvm2 solves your original problem - which I still don't seem to understand - I'm just saying you need to look at how lvm2 is ordering ioctls with loads & resumes of targets when making snapshot. IMHO old snapshot is quite complicated and maybe you should take a look at this provisioning support - especially if you think in terms of having lots of snapshot of single master volume - usage of old-snapshot target is pretty much dead road.... Regards Zdenek