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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-701) Lock-less commits
Date Wed, 01 Nov 2006 18:35:19 GMT
    [ http://issues.apache.org/jira/browse/LUCENE-701?page=comments#action_12446342 ] 
            
Michael McCandless commented on LUCENE-701:
-------------------------------------------

Thanks Yonik for looking at this!

> Could you elaborate on how the backward compatibility works w.r.t. modifying an old index?
> How do versioned norms & del files mix with older unversion files?

OK, first off, the only file whose contents and name are changed is
the "segments_N" file.  Then, the only files whose name (but not
contents) is changed are the "_X_N.del" deletes file, and "_X_N.sZ"
separate norms files).  Finally, the only file that is deleted is
deletable.  All other segments files are unchanged.

The unit test I added for this (TestBackwardsCompatibility) un-zips a
pre-lockless index (I have one zip file for CFS format and one for
non-CFS) and then runs various tests: adding docs, deleting a doc,
setting a norm for a doc, searching, etc., verifying that the results
are then as expected.

On opening an pre-lockless index, first we see only a "segments" file
with no _N extension, and record its generation as 0.  Second, since
that file's contents doesn't lead with format lockless code (-2), we
know to load the old segments info format that does not contain the
"del" gen, "separate norm" gen nor the "isCompundFile" marker.

When loading each Segmentinfo, since the format of the segments files
was old, we record this with "preLockless = true" and set delGen and
isCompoundFile to 0 and normGen to null.  0 means "must check
filesystem for existence".  The various methods (hasSeparateNorms,
hasDeletions, etc.) know to handle these "0" cases as falling back to
filesystem existence checks of the existing naming (ie, _X.del).  I
tried to capture / contain all of this inside the methods of
SegmentInfo.

Now, when a writer commits to this pre-lockless index, we write in the
new (format lockless) format and to the file "segments_1".  (Actually,
with compound file turned on, we then make a .cfs file and commit
again to "segments_2").  This file will reference all of the old
segments (except eg those deleted by a merge) plus the one new
segment.

The old segments have written the "0"'s for delGen, normGen (null is
written as -1 length), isCompoundFile so that on re-loading this
segments_2 file these segments remain pre-lockless.  The SegmentInfo
for the new segment file will have isCompoundFile=1 (meaning it is a
compound file), delGen=-1 (there are no separate deletes yet) and
normGen=null (there are no separate norms yet).  When normGen is null,
we look at "preLockless" to differentiate whether this means there are
no separate norms at all for any fields, or, this segment is pre
lockless and therefore we must fallback to the filesystem check.

If a delete or setNorm call takes place against an old segment format,
we will at that time create "generation 1" for that file.  This means
you can have an old segment whose separate del file is still
"generation 0" (you have to check for existence of _X.del) but whose
separate norms generations are known (or, only certain fields are
known and the others are "generation 0" and require filesystem check).
This means an "old" segment file could become "slightly newer" as
norm/del changes are made against it.

So an index can have mixed old/new segments.  The SegmentInfo for each
segment keeps track of old/new (and tries to hide these implementation
details under its methods) with delGen, normGen, isCompoundFile and
preLockless (which is derived from isCompoundFile).

Once an optimize is done, or, all old segments have been merged away,
then all segments are now the lockless format.



> 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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