On Nov 13, 2006, at 8:10 AM, Øyvind Stegard wrote:
I've searched the list and have found many references to problems when
using Lucene over NFS. Mostly because of file-based locking, which
doesn't work all that well for many NFS installations. I'm under the
impression that the core locking logic between writers and/or readers
hasn't changed in a significant way between Lucene 1.4 and 2.0 (?). I
guess this means NFS is still problematic ?
Unfortunately it all depends on the reliability of the NFS drivers in
the OS, and the kind of filers you are using. If the environment
isn't too busy, NFS lockd *may* work on some systems, but it usually
ends up collapsing under load.
From there you have to hand craft some C code to create lock files,
and what works again depends on your system. On some systems doing an
exclusive create will work (can only be expected to work on version 3
mounts), but then local caches will bite you, so you end up having to
disable the directory cache, assuming your system supports such an
option. That failing, creating locks as symlinks to unique temporary
files that don't exist will usually blow through the cache and work
ok. This of course doesn't rule out problems in the NFS
implementation that show up under heavy load, and allow more than one
machine to think it has the lock. You also have to include some code
to sensibly expire locks left from crashes.
We are considering a model where a single node updates the search
index
according to changes in the repository (only one physical index for
the
entire cluster) while multiple other nodes can search the very same
index over NFS (read-only). But I guess there is a need for a single
lock-directory shared and writable between all nodes, and that this
makes NFS-usage difficult ?
The fact that only a single node will be doing writes greatly
improves the chances of this working. I don't recall whether readers
ever check for locks, it's best if that can be avoided. I know that
it's safe to write the new indexes since they aren't being referred
to by the segments file, but I'm not sure what sequence of operations
are used when re-writing the segments file. I think unlinking the old
segments file and using a rename to put the new one in place is
probably the safest bet.
Peter
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