Date: Thu, 11 Feb 2010 15:15:49 -0500 From: Pierre-Luc Drouin <pldrouin@pldrouin.net> To: Matthew Seaman <m.seaman@infracaninophile.co.uk> Cc: freebsd-questions@freebsd.org Subject: Re: Memory disk "a la mfsroot"? Message-ID: <4B746575.4010501@pldrouin.net> In-Reply-To: <4B745834.1010309@infracaninophile.co.uk> References: <4B7419F5.7050602@pldrouin.net> <4B741E56.8010002@infracaninophile.co.uk> <20100211153925.7b88844d@gumby.homeunix.com> <4B74283B.1070903@infracaninophile.co.uk> <4B743A16.9020201@pldrouin.net> <4B745834.1010309@infracaninophile.co.uk>
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Matthew Seaman wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > On 11/02/2010 17:10, Pierre-Luc Drouin wrote: > >>> Actually, I was thinking more along the lines of mounting a .iso as a >>> cd9660 filesystem. Which won't muck up the underlying .iso, but only >>> because it's read-only. You could mount a FFS image read-only in >>> exactly the same way -- I think there's a 'last mounted on' field in the >>> backing file image that will be updated if the it is writable (even if >>> the fs itself is mounted ro) but that's not the right answer either. >>> >>> Basically, you're going to have to mount and initialise as two separate >>> operations as far as I can see. >>> >>> >> By this do you mean that I would need to copy the whole content of the >> read-only filesystem to the memory disk? >> > > Yes. > > >> I looked at the man page for mount_unionfs and there is a big warning >> saying that it is a bad idea to use it, so I guess I will pass on this >> solution... >> > > Ah -- that warning is probably a bit more alarming than it really > needs to be nowadays. unionfs can be used pretty effectively for many > purposes. Try it and see what happens is the best policy. > > >> What I am trying to do basically is to mount a filesystem from a CD but >> I want to use a memory disk to allow write operations. I would basically >> want the filesystem to behave like a regular read-write filesystem, but >> not have to copy everything into a memory disk. What does "mfs_root" do >> exactly in the official FreeBSD boot CDs? Does it copy the content of >> mfsroot.gz into a memory disk? That filesystem is so small that I guess >> it can be copied without any problem... >> > > mfs_root does exactly that: it creates a memory based filesystem and > then expands a tarball of the system into it. > > One approach you might consider is mounting your CD image read-only as > per usual, but creating memory-based /tmp and /var partitions[*]. Most > of the usual root and /usr filesystems don't need to be read-write at > all. There are only a few special locations that do and those will need > special handling. You will need to initialise your memory-backed /var > partiton by expanding a skeleton structure into it, but that's going to > be pretty small really. You will also need to make provision for editing > various files under /etc -- you might be able to create a /var/etc and > replace the real files in /etc with symlinks to copies in /var/etc. > Possibly. Or you could just have /var/etc nullfs mounted on top of /etc. > > I've read reports from people setting up such things -- a while back > now, but as far as I recall they were on the whole successful. > > Cheers, > > Matthew > > [*] There are already scripts to do this sort of thing within the base > system, although primarily aimed at diskless systems. > > - -- > Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard > Flat 3 > PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate > Kent, CT11 9PW > -----BEGIN PGP SIGNATURE----- > Version: GnuPG/MacGPG2 v2.0.14 (Darwin) > Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ > > iEYEARECAAYFAkt0WDQACgkQ8Mjk52CukIyWgwCeMHjE7VsdHx5HLGQw75SplJ9v > 6zUAni7BZsLYsp2qGSFUUMmkjpVMYomt > =4SwV > -----END PGP SIGNATURE----- > Thanks, so I think I will do the following: 1-Create a mfsroot.gz with empty /usr, /var, /boot/kernel and /boot/modules directories and have it mounted through loader.conf 2-Create a rc.d script that is called immediately after rc.d/conf and that does the following: -Mount /usr using mdmfs and an independent backing file on the CD (so this way the whole content of /usr would not be loaded into RAM) -Create a 500MB memory disk and mount it in /memdisk -Create /memdisk/{var,tmp,root,home,usr} -mount_nullfs /memdisk/var and /memdisk/tmp on /var and /tmp respectively -copy the content of /root and /home (i.e. just a few config files) to their respective directories in /memdisk -mount_nulls /memdisk/root and /memdisk/home on /root and /home respectively -mount_unionfs /memdisk/usr over /usr 3-use populate_var="YES" in rc.conf to automatically populate the /var directory So using this I would have a read-write file system without loading the whole content of /usr into RAM, I would minimize memory usage by regrouping all the writable directories into a single memory disk and I would also minimize the amount of data that is copied from the CD to the memory. Does it sound like a good plan? I read something about a limit of 45MB for mfsroot.gz to prevent kernel crashing. Have you ever heard about this? Thanks!
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