Date: Mon, 23 Jan 2012 14:42:19 +0100 From: Andreas Nilsson <andrnils@gmail.com> To: Johannes Totz <johannes@jo-t.de> Cc: freebsd-fs@freebsd.org Subject: Re: Booting from zfs snapshot Message-ID: <CAPS9%2BSvA8c=uY7b%2BeKMrA2CnFS4Kg1ccxgaVYm9RSZy1GQ6ChA@mail.gmail.com> In-Reply-To: <jfjlhq$p77$1@dough.gmane.org> References: <CAPS9%2BSsyvw=Day%2B1XC8HmzWQG3M=GZE=Nt-qaEQiQTPYME9MrA@mail.gmail.com> <jfgv8j$82s$1@dough.gmane.org> <CAPS9%2BSvtYZt1m26jCqRH6c3w_aB3hWt56ZFoYJAw_cEu2NANzg@mail.gmail.com> <jfjlhq$p77$1@dough.gmane.org>
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On Mon, Jan 23, 2012 at 1:58 PM, Johannes Totz <johannes@jo-t.de> wrote: > On 22/01/2012 20:15, Andreas Nilsson wrote: > >> On Sun, Jan 22, 2012 at 1:25 PM, Johannes Totz<johannes@jo-t.de> wrote: >> >> On 19/01/2012 16:26, Andreas Nilsson wrote: >>> >>> Hello, >>>> >>>> I'm trying to wrap my head around the process of booting from a zfs >>>> snapshot. I have hit a few roadblocks, which I hope this is the adequate >>>> list to post to regarding those. >>>> >>>> A short note on what I'm trying to achieve might be in order. In short: >>>> a >>>> nanobsd system on zfs only. I want to boot from a snapshot so that when >>>> I >>>> push out an upgrade with zfs send, I want the root filesystem to remain >>>> unchanged. >>>> >>>> The problems I've hit so far: >>>> *1 Making the zpool.cache file available >>>> *2 Having / mount via entry in fstab. >>>> >>>> >>> FWIW, I dont use any fstab for my zfs-only machine. Works perfectly fine >>> with mountpoint property. >>> >> >> >> It was just a small hope that the boot would continue with the filesystem >> pointed out by the bootfs property. I'm setting vfs.root.mountfrom in >> loader.conf now. >> >> >> >>> >>> *1: The zpool.cache is needed to autoimport a pool as I understand it. >>> Is >>> >>>> there a way to force the kernel to import a pool during bootup even >>>> though >>>> no zpool.cache is around? What does this file actually contain? >>>> >>>> I made an experiment and booted a disk with zfs root from machine a in >>>> machine b and that worked. I did partition the disk with gpart using a >>>> gpt >>>> scheme, and labeled the partition on which the pool resides as os, and >>>> upon >>>> creation of the zpool used gpt/os as device. Does this mean that as long >>>> as >>>> gpt/os is available, any machine boot this disk will have the zpool >>>> autoimported? >>>> >>>> >>> Not quite sure I understand you here. Just a note: booting kernel and >>> mounting root fs are two different things. the *zfsloader will happily >>> load >>> the kernel off a pool and boot it but mounting root might fail later (I >>> guess if no cachefile is present?). >>> >>> Sorry for being unclear. What I experience is exactly what you're >> describing: gptzfsloader loads the kernel and runs it just fine, but >> mounting root fails due to missing zpool.cache. >> >> I'm looking for a way to have the pool imported without the zpool.cache >> >> >> >>> >>> *2: Having a line like >>>> tank/root/8.2-RELEASE-p5@ro / zfs ro 0 0 >>>> in fstab causes mount to throw an error and leave me in single user >>>> mode, >>>> when the system is booted however mount can mount a zfs snapshot just >>>> fine. >>>> Setting vfs.root.mountfrom in loader.conf works just fine though. >>>> >>>> >>> The above I still think is rather strange: setting vfs.root.mountfrom >> to a >> snapshot ( given that the snapshot has the zpool.cache file) works, but >> not >> having the corresponding line in fstab. >> >> >>> >>> -- >>> Sent from my<insert random gadget here> >>> >>> >> What I'm seeking a solution to is this: Boot several machines from one zfs >> snapshot. Since the stream from zfs send is also used to do the initial >> install there is no easy way to get the correct zpool.cache file in the >> snapshot. I guess one possible way to tackle this problem is to modify >> zpool so that all pools get created with the same id. >> > > There's a way to pre-load a zpool.cache via the loader (from an > alternative location). Can't remember the correct variables to set right > now... > > As for w/o any cache file... no idea. > > Ok, I'll research that. If it could be loaded from another disk/partition I could really use it :) It is of course possible to edit the cachefile property of the zpool, but I think one can only set it to something relative the dataset from which one boots. Regards Andreas
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