Mounting two different disk slices on the same mount point

Matthew Seaman m.seaman at
Sat Aug 7 05:00:24 PDT 2004

On Sat, Aug 07, 2004 at 01:36:39PM +0300, Odhiambo Washington wrote:
> Hello FreeBSD gurus,
> Some linux guy gave me a strange idea:
> You have two disks, /dev/da1s1e and /dev/da2s1e and I mount the
> two to a single mount point.
> Do I end up with the total capacity of the two disks on the said
> mount point?
> To be a bit clear, da1 is, say, 36GB and da2 is 72GB and I mount
> as above to /mnt
> Does /mnt now have 108GB?
> What are the dangers of doing this?
> How does the system use the disks?
> I haven't tested this anyway.
> Is this something sane?
> Clues will be highly appreciated.

That sounds a bit muddled.  If you try mounting two partitions
literally in the same place, either one will overlay the other so you
can only access the last mounted partition, or you'll get an error
message and fail to mount the second one. (Which occurs depends on the
filesystem type).

What I think your Linux using friends were thinking of, even if they
were unclear on the concept, is some form of logical volume

Under FreeBSD, you could use vinum(8) to create a concat volume from
the two partitions, which would give you give you a filesystem the
size of the sum of both components.  That's not new functionality.
It's been available in FreeBSD for ages (since 2.x I believe) through
vinum(8) or one of it's predecessors like ccd(4).



Dr Matthew J Seaman MA, D.Phil.                       26 The Paddocks
                                                      Savill Way
PGP:         Marlow
Tel: +44 1628 476614                                  Bucks., SL7 1TH UK
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