md device backing files on nfs mounts?

Lowell Gilbert freebsd-questions-local at
Thu Oct 6 10:18:56 PDT 2005

"Chad Leigh -- Shire.Net LLC" <chad at> writes:

> On Oct 6, 2005, at 10:51 AM, Lowell Gilbert wrote:
> > "Chad Leigh -- Shire.Net LLC" <chad at> writes:
> >
> >
> >> Can the files that back an md device be resident on an nfs mount?
> >>
> >> I run some jails with each having its own root based on an md
> >> device.
> >> I am thinking of having a backend nfs server have all the  storage
> >> and
> >> serve it to various front end servers.  If one front end  server went
> >> down I could easily bring it up on another one.  Kind of  poor-man's
> >> redundancy
> >>
> >
> > How do you get the NFS mount before you have a root?
> Just the jails would be on md devices on the nfs mounted  filesystem.
> Ie, main computer boots normally, mounts nfs  filesystem, then mounts
> md devices backed by files on the nfs  fielsystem

Yes, okay, that seems obvious now.  Maybe I need more caffeine.

I just tried it without the jails and was able to mount and use the
filesystem.  The permissions *are* a little tricky, because root is
(as usual) mapped to nobody on my NFS mounts, and needs write access
on the backing file.

I did the procedure from the last example in the mdconfig(8) manual,
and it worked just like on a local disk.  There might have been some
anomalies with the output of ls(1), but I can't reproduce those now.

You might want to try measuring performance if you implement this; it
could have some strange interactions with VM and buffering.

Clever idea, by the way.

> Chad
> > Also, I suspect the permissions might be a little tricky...
> >
> > Some kind of netbooting might work.

Lowell Gilbert, embedded/networking software engineer, Boston area

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