Memory disk "a la mfsroot"?

Pierre-Luc Drouin pldrouin at
Thu Feb 11 17:10:50 UTC 2010

Matthew Seaman wrote:
> Hash: SHA1
> On 11/02/2010 15:39, RW wrote:
>> On Thu, 11 Feb 2010 15:12:22 +0000
>> Matthew Seaman <m.seaman at> wrote:
>>> On 11/02/2010 14:53, Pierre-Luc Drouin wrote:
>>>> I would like to know if there is a mount command that allows to
>>>> create a memory disk that can be initialized from a file. What I am
>>>> looking for is something like mount_mfs -F, but that does not
>>>> modify the actual file. I know what I could easily to this by
>>>> copying the content of the file to the memory disk, but I am
>>>> looking for a solution that can be configured via fstab.
>>> Yes.  See mdconfig(8) -- there are examples in there of exactly what
>>> you want to do.
>> I don't think covers what he is asking for. I think you would need a
>> union filsystem that overlays a swap-backed filesystem on top of a
>> file-backed filesystem - if that's possible.
> 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?

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 

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...

> 	Cheers,
> 	Matthew
> - -- 
> Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
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