From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <5204B905.5040702@redhat.com> Date: Fri, 09 Aug 2013 11:40:21 +0200 From: Zdenek Kabelac MIME-Version: 1.0 References: <20130806173719.GB15184@mail.waldi.eu.org> <520211BB.2040301@pse-consulting.de> <5202164B.5010302@redhat.com> <52028170.1010000@pse-consulting.de> <52036C86.3040702@redhat.com> <5204A0EF.7010803@pse-consulting.de> In-Reply-To: <5204A0EF.7010803@pse-consulting.de> Content-Transfer-Encoding: 7bit Subject: Re: [linux-lvm] Missing error handling in lv_snapshot_remove Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: LVM general discussion and development Cc: Andreas Pflug Dne 9.8.2013 09:57, Andreas Pflug napsal(a): > Am 08.08.13 12:01, schrieb Zdenek Kabelac: >> Dne 7.8.2013 19:18, Andreas Pflug napsal(a): >>> On 08/07/13 11:41, Zdenek Kabelac wrote: >>>> Dne 7.8.2013 11:22, Andreas Pflug napsal(a): >>>>> Am 06.08.13 19:37, schrieb Bastian Blank: >>>>>> Hi >>>>>> >>>>>> I tried to tackle a particular bug that shows up in Debian for >>>>>> some time >>>>>> now. Some blamed the udev rules and I still can't completely rule >>>>>> them >>>>>> out. But this triggers a much worse bug in the error cleanup of the >>>>>> snapshot remove. I reproduced this with Debian/Linux 3.2.46/LVM >>>>>> 2.02.99 >>>>>> without udevd running and Fedora 19/LVM 2.02.98-10.fc19. >>>>>> >>>>>> On snapshot removal, LVM first converts the device into a regular LV >>>>>> (lv_remove_snapshot) and in a second step removes this LV >>>>>> (lv_remove_single). Is there a reason for this two step removal? An >>>>>> error during removal leaves a non-snapshot LV behind. >>>>> Ah, this explains why sometimes my backup stops: I take a snapshot, >>>>> rsync the stuff and remove the snapshot with a daily cron job, but I >>>>> observed twice that a non-snapshot volume named like a backup snapshot >>>>> was lingering around, preventing the script to work. So this is no >>>>> exotic corner case, but happens in real life. >>>>> >>>>> I observe this since I dist-upgraded to wheezy. >>>>> >>>> >>>> Because Debian is using non-upstream udev rules. >>>> >>>> With upstream udev rules with standard real-life use, this situation >>>> cannot happen - since these rules are constructed to play better with >>>> udev WATCH rule. >>> >>> Hm, does udev play a role on this at all? Without having dived the >>> code, I'd >>> assume udev has only to do with creation and deletion of /dev/mapper/... >>> and/or /dev/vgname/... devices (upon lvchange -aX), but not with lvm >>> metadata >>> manipulation. >> >> >> Udev attempts to update it device database after any change event >> (you could observe its work with udevadm monitor) >> >> So in your case - you unmount filesystem -> close device -> fires >> WATCH event with some randomly delayed (systemd)udevd scan machism - >> so in unpredictable moment blkid opens device and scans its sectors >> (keeping device open and interfering with deactivate operation). For >> this short-time opens there is now built-in retry which tries to >> deactivate device several times when it's known device is not mounted. > > So in order to harden my script against this problem, I should > deactivate the volume explicitely, wait a while and then remove it? If you call 'udevadm settle' after umount -- it will wait till udev finishes its work. However recent lvm2 has the 'retry' loop built-in - so it should not be needed if the proper udev rules are in place. Zdenek