The quick answer is: NFS is still problematic in Lucene 2.0.
The longer answer is: we'd like to fix this, but it's not fully fixed
yet. You can see here:
http://issues.apache.org/jira/browse/LUCENE-673
for gory details.
There are at least two different problems with NFS (spelled out in the
above issue):
* Intermittant IOException on instantiating a reader.
This is in fact [surprisingly] not due to locking, at least in my
testing. The unreleased version of Lucene now supports native
locks (through java.nio.*), but even when using native locks I can
still reproduce this error in my testing.
The good news is: the lockless commits patch (which is not yet
committed but I think close):
http://issues.apache.org/jira/browse/LUCENE-701
resolves this issue. Lockless commits also makes readers entirely
read only, so your read-only NFS mount for readers becomes
possible.
* "Stale NFS handle" IOException when searching.
Lucene's readers provide "point in time" searching: once open,
a reader searches the snapshot of the index as of the point it was
open.
Unfortunately, the implementation of this feature currently relies
on the filesystem to provide access to files even after they are
deleted. NFS makes no such guarantee.
This means on searching you have to catch this exception and then
close & open a new searcher.
I think it would make sense to change how Lucene implements point
in time searching so we don't rely on filesystem semantics. But
this is a ways off.
I'm hopeful that with lockless commits, and then with the caveat of
closing/opening your searchers on hitting "Stale NFS handle" during
searching (until we can change how "point in time" searching is
implemented), that Lucene will work fine over NFS.
Anyway, in the meantime, one good workaround is to either use Solr:
http://incubator.apache.org/solr/
directly, or, borrow its approach. With Solr, a writer writes to the
index and periodically (at a known safe time) takes a snapshot, and
then readers only read from the current snapshot.
Mike
Peter A. Friend wrote:
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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