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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] [Created] (DERBY-5738) SYSCS_UTIL.SYSCS_INPLACE_COMPRESS_TABLE only returns to the OS pages in the last extent
Date Wed, 02 May 2012 23:09:48 GMT
Mike Matrigali created DERBY-5738:

             Summary: SYSCS_UTIL.SYSCS_INPLACE_COMPRESS_TABLE only returns to the OS pages
in the last extent
                 Key: DERBY-5738
                 URL: https://issues.apache.org/jira/browse/DERBY-5738
             Project: Derby
          Issue Type: Bug
    Affects Versions:,,,,,,,
            Reporter: Mike Matrigali

It looks like inplace compress only gives back space to the OS from the empty pages in the
last extent of the file.  If the defrag and purge phases are
run they can reclaim internal space in the form of free pages to be used for future inserts,
but free pages before last extent are not returned to 

Only current workaround is to use SYSCS_UTIL.SYSCS_COMPRESS_TABLE instead.

In filecontainer.java!compressContainer there is a comment with steps to be performed, but
no code exists for step 5 and 6:

      This is outline of the algorithm used in compressing the container.

      Until a non free page is found loop, in each loop return to the OS
         all space at the end of the container occupied by free pages, including
         the allocation page itself if all of it's pages are free.

      1) Find last 2 allocation pages in container (last if there is only one).
      2) invalidate the allocation information cached by the container.
         Without the cache no page can be gotten from the container.  Pages
         already in the page cache are not affected.  Thus by latching the
         allocPage and invalidating the allocation cache, this NTT blocks out
         all page gets from this container until it commits.
      3) the allocPage determines which pages can be released to the OS,
         mark that in its data structure (the alloc extent).  Mark the
         contiguous block of nallocated/free pages at the end of the file
         as unallocated.  This change is associated with the NTT.
      4) The NTT calls the OS to deallocate the space from the file.  Note
         that the system can handle being booted and asked to get an allocated
         page which is past end of file, it just extends the file automatically.
      5) If freeing all space on the alloc page, and there is more than one
         alloc page, then free the alloc page - this requires an update to the
         previous alloc page which the loop has kept latched also.
      6) if the last alloc page was deleted, restart loop at #1

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