[Unionfs] Using UnionFS on many many filesystems

[Erez Zadok]

In message <6F79B9DB472D0C4F82EC633CBDEDAAE9618627 at elessar.cc.w2k.vt.edu>, "Fabian, Andy" writes:
> Are there any hard limits on the number of filesystems that can be
> joined concurrently into a single UnionFS filesystem?  Are there any
> performance characteristics that drop more than linearly as the number
> of source filesystems increase?
> 
> I'm thinking of using UnionFS in a backup system, to join potentially
> hundreds of incremental backups together.  Is there any reason this
> shouldn't work?

Andrew,

Unionfs 2.x has one internal limit of 128 branches in can handle.  This
limit can be changed in fs/unionfs/union.h:

/* maximum number of branches we support, to avoid memory blowup */
#define UNIONFS_MAX_BRANCHES	128

The reason for the limit, as the comment above states, is to avoid an
unbounded memory consumption.  Imagine someone inserting more and more
branches into an existing union -- eventually you'll run out of physical
kernel memory.

So if you think you need more branches, try to change the above macro and
recompile and let me know.

Erez.