On Sunday, August 26, 2007 at 16:36:26 CEST, Rainer J.H. Brandt wrote:
> Ronald Kuehn writes:
> > No. You can neither access ZFS nor UFS in that way. Only one
> > host can mount the file system at the same time (read/write or
> > read-only doesn't matter here).
>
> I can see why you wouldn't recommend trying this with UFS
> (only one host knows which data has been committed to the disk),
> but is it really impossible?
>
> I don't see why multiple UFS mounts wouldn't work, if only one
> of them has write access. Can you elaborate?
Hi,
UFS wasn't designed as a shared file system. The kernel
always assumes it is the only party accessing or modifying
any on-disk data structures. With that premise it uses caching
quite heavily. The view of the file system (cached structures + on-disk
state) is consistent. The on-disk state alone isn't while the
file system is mounted. Any other system accessing the on-disk
state w/o taking into consideration the data cached on the original
host will probably see inconsistencies. This will lead to data corruption
and panics. If only one system mounts the file system read/write
and other hosts only mount it read-only the read-only hosts will
get an inconsistent view of the file system because they don't know
what's in the cache of the r/w host.
These approaches exist to solve this problem:
- Only allow one host to directly access the file system. Other
systems access it by talking over the network to this host:
+ NFS
+ the pxfs layer of Sun Cluster (global file system)
- Use a file system designed with some kind of co-ordination for parallel
access to the on-disk data structures built in:
+ QFS (Shared mode uses a meta data server on one host to
manage the right to access certain parts of the on-disk structures.
The operation on the data itself then takes place over the storage
path. In that case multiple systems can modify on-disk structures
directly. They only need to ask the meta data server for permission.)
I hope that helps,
Ronald
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