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Date: Thu, 2 Nov 2006 09:43:22 -0800 (PST)
From: "Yonik Seeley (JIRA)" <jira@apache.org>
To: java-dev@lucene.apache.org
Subject: [jira] Commented: (LUCENE-701) Lock-less commits
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    [ http://issues.apache.org/jira/browse/LUCENE-701?page=comments#action_12446665 ] 
            
Yonik Seeley commented on LUCENE-701:
-------------------------------------

> > In the future, it might be nice if there was an option to disable
> > segments.gen to be more friendly to write-once filesystems like
> > HDFS.
> I think this makes sense. I will add control over this on the next
> iteration of the patch. 

Just to be clear, I didn't mean that I thought it was needed now... 
there is another place in Lucene that prevents write-once from working (segment file lengths at the beginning IIRC).

When this option is added, perhaps the configuration name should be generic and not tied to the implementation specifics that could change more frequently?  Something like WRITE_ONCE or setWriteOnce()?

> Lock-less commits
> -----------------
>
>                 Key: LUCENE-701
>                 URL: http://issues.apache.org/jira/browse/LUCENE-701
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>    Affects Versions: 2.1
>            Reporter: Michael McCandless
>         Assigned To: Michael McCandless
>            Priority: Minor
>         Attachments: index.prelockless.cfs.zip, index.prelockless.nocfs.zip, lockless-commits-patch.txt
>
>
> This is a patch based on discussion a while back on lucene-dev:
>     http://mail-archives.apache.org/mod_mbox/lucene-java-dev/200608.mbox/%3c44E5B16D.4010805@mikemccandless.com%3e
> The approach is a small modification over the original discussion (see
> Retry Logic below).  It works correctly in all my cross-machine test
> case, but I want to open it up for feedback, testing by
> users/developers in more diverse environments, etc.
> This is a small change to how lucene stores its index that enables
> elimination of the commit lock entirely.  The write lock still
> remains.
> Of the two, the commit lock has been more troublesome for users since
> it typically serves an active role in production.  Whereas the write
> lock is usually more of a design check to make sure you only have one
> writer against the index at a time.
> The basic idea is that filenames are never reused ("write once"),
> meaning, a writer never writes to a file that a reader may be reading
> (there is one exception: the segments.gen file; see "RETRY LOGIC"
> below).  Instead it writes to generational files, ie, segments_1, then
> segments_2, etc.  Besides the segments file, the .del files and norm
> files (.sX suffix) are also now generational.  A generation is stored
> as an "_N" suffix before the file extension (eg, _p_4.s0 is the
> separate norms file for segment "p", generation 4).
> One important benefit of this is it avoids files contents caching
> entirely (the likely cause of errors when readers open an index
> mounted on NFS) since the file is always a new file.
> With this patch I can reliably instantiate readers over NFS when a
> writer is writing to the index.  However, with NFS, you are still forced to
> refresh your reader once a writer has committed because "point in
> time" searching doesn't work over NFS (see LUCENE-673 ).
> The changes are fully backwards compatible: you can open an old index
> for searching, or to add/delete docs, etc.  I've added a new unit test
> to test these cases.
> All units test pass, and I've added a number of additional unit tests,
> some of which fail on WIN32 in the current lucene but pass with this
> patch.  The "fileformats.xml" has been updated to describe the changes
> to the files (but XXX references need to be fixed before committing).
> There are some other important benefits:
>   * Readers are now entirely read-only.
>   * Readers no longer block one another (false contention) on
>     initialization.
>   * On hitting contention, we immediately retry instead of a fixed
>     (default 1.0 second now) pause.
>   * No file renaming is ever done.  File renaming has caused sneaky
>     access denied errors on WIN32 (see LUCENE-665 ).  (Yonik, I used
>     your approach here to not rename the segments_N file(try
>     segments_(N-1) on hitting IOException on segments_N): the separate
>     ".done" file did not work reliably under very high stress testing
>     when a directory listing was not "point in time").
>   * On WIN32, you can now call IndexReader.setNorm() even if other
>     readers have the index open (fixes a pre-existing minor bug in
>     Lucene).
>   * On WIN32, You can now create an IndexWriter with create=true even
>     if readers have the index open (eg see
>     www.gossamer-threads.com/lists/lucene/java-user/39265) .
> Here's an overview of the changes:
>   * Every commit writes to the next segments_(N+1).
>   * Loading the segments_N file (& opening the segments) now requires
>     retry logic.  I've captured this logic into a new static class:
>     SegmentInfos.FindSegmentsFile.  All places that need to do
>     something on the current segments file now use this class.
>   * No more deletable file.  Instead, the writer computes what's
>     deletable on instantiation and updates this in memory whenever
>     files can be deleted (ie, when it commits).  Created a common
>     class index.IndexFileDeleter shared by reader & writer, to manage
>     deletes.
>   * Storing more information into segments info file: whether it has
>     separate deletes (and which generation), whether it has separate
>     norms, per field (and which generation), whether it's compound or
>     not.  This is instead of relying on IO operations (file exists
>     calls).  Note that this fixes the current misleading
>     FileNotFoundException users now see when an _X.cfs file is missing
>     (eg http://www.nabble.com/FileNotFound-Exception-t6987.html).
>   * Fixed some small things about RAMDirectory that were not
>     filesystem-like (eg opening a non-existent IndexInput failed to
>     raise IOException; renames were not atomic).  I added a stress
>     test against a RAMDirectory (1 writer thread & 2 reader threads)
>     that uncovered these.
>   * Added option to not remove old files when create=true on creating
>     FSDirectory; this is so the writer can do its own [more
>     sophisticated because it retries on errors] removal.
>   * Removed all references to commit lock, COMMIT_LOCK_TIMEOUT, etc.
>     (This is an API change).
>   * Extended index/IndexFileNames.java and index/IndexFileNameFilter.java
>     with logic for computing generational file names.
>   * Changed index/IndexFileNameFilter.java to use a HashSet to check
>     file extentsions for better performance.
>   * Fixed the test case TestIndexReader.testLastModified: it was
>     incorrectly (I think?) comparing lastModified to version, of the
>     index.  I fixed that and then added a new test case for version.
> Retry Logic (in index/SegmentInfos.java)
> If a reader tries to load the segments just as a writer is committing,
> it may hit an IOException.  This is just normal contention.  In
> current Lucene contention causes a [default] 1.0 second pause then
> retry.  With lock-less the contention causes no added delay beyond the
> time to retry.
> When this happens, we first try segments_(N-1) if present, because it
> could be segments_N is still being written.  If that fails, we
> re-check to see if there is now a newer segments_M where M > N and
> advance if so.  Else we retry segments_N once more (since it could be
> it was in process previously but must now be complete since
> segments_(N-1) did not load).
> In order to find the current segments_N file, I list the directory and
> take the biggest segments_N that exists.
> However, under extreme stress testing (5 threads just opening &
> closing readers over and over), on one platform (OS X) I found that
> the directory listing can be incorrect (stale) by up to 1.0 seconds.
> This means the listing will show a segments_N file but that file does
> not exist (fileExists() returns false).
> In order to handle this (and other such platforms), I switched to a
> hybrid approach (originally proposed by Doron Cohen in the original
> thread): on committing, the writer writes to a file "segments.gen" the
> generation it just committed.  It writes 2 identical longs into this
> file.  The retry logic, on detecting that the directory listing is
> stale falls back to the contents of this file.  If that file is
> consistent (the two longs are identical), and, the generation is
> indeed newer than the dir listing, it will use that.
> Finally, if this approach is also stale, we fallback to stepping
> through sequential generations (up to a maximum # tries).  If all 3
> methods fail, we throw the original exception we hit.
> I added a static method SegmentInfos.setInfoStream() which will print
> details of retry attempts.  In the patch it's set to System.out right
> now (we should turn off before a real commit) so if there are problems
> we can see what retry logic had done.

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