Author image Marvin Humphrey
and 1 contributors

NAME

KinoSearch::Docs::NFS - Managing invindexes on NFS.

Problem

When a file is no longer in use by an index, InvIndexer attempts to delete it as part of a cleanup routine triggered by a call to finish(). It is possible that at the moment an InvIndexer attempts to delete files that it no longer thinks are needed, a Searcher is in fact using them. This is particularly likely in a persistent environment (mod_perl, FastCGI, etc), where Searchers are cached and reused. Ordinarily, this is not a problem.

On a typical Unix volume, the file will be deleted in name only: any process which holds an open filehandle against that file will continue to have access, and the file won't actually get vaporized until the last filehandle is cleared. Thanks to "delete on last close semantics", an InvIndexer can't truly delete the file out from underneath an active Searcher.

On Windows, KinoSearch will attempt the file deletion, but an error will occur if any process holds an open handle. That's fine; InvIndexer runs these unlink() calls within an eval block, and if the attempt fails it will just try again the next time around.

On NFS, however, the system breaks, because NFS allows files to be deleted out from underneath an active process. Should this happen, the unlucky Searcher will crash with a "Stale NFS Filehandle" exception.

Workaround

The workaround is to maintain multiple copies of an index.

  1. Start state: Two identical indexes, A and B. Searchers are active against A.

  2. An InvIndexer updates B. As it finishes, it deletes any unneeded files.

  3. Searchers are switched to using index B.

  4. Synchronize index A so that it looks exactly the same as B. You could just delete A and copy B onto where it was, but there are more efficient ways of handling the synchronization since not all the files change -- e.g. rsync, pax.

  5. An InvIndexer updates A. As it finishes, it deletes any unused files.

  6. Etc...

COPYRIGHT

Copyright 2005-2007 Marvin Humphrey

LICENSE, DISCLAIMER, BUGS, etc.

See KinoSearch version 0.20.