On Sunday, August 26, 2007 at 17:47:32 CEST, Rainer J.H. Brandt wrote: > Ronald Kuehn writes: > > 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 > > Yes, thank you for confirming what I said. > > So it is possible, but not recommended, because I must take care > not to read from files for which buffers haven't been flushed yet.
It is technically possible to mount the file system on more than one system, but it _will_ lead to data corruption and panics. Just making sure buffers for a file have been flushed to disk on the writer is _not_ enough. So it is not only not recommended, it is practically not possible to do such a configuration in a safe way. There is no way to force the read-only host to only read the data when they are consistent. Even on a lower level, writes to the file system are not atomic. When the read-only host picks up data while the other hosts update is not complete it will get random inconsistencies instead of correct meta-data. So as a summary: No way to do that with current UFS. Ronald _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
