[Xapian-tickets] [Xapian] #284: occasional !DatabaseModifiedErrors

[Xapian]

#284: occasional !DatabaseModifiedErrors
---------------------------+------------------------------------------------
 Reporter:  mrks           |       Owner:  olly    
     Type:  defect         |      Status:  new     
 Priority:  normal         |   Milestone:          
Component:  Backend-Flint  |     Version:  SVN HEAD
 Severity:  normal         |    Keywords:          
Blockedby:                 |    Platform:  Linux   
 Blocking:                 |  
---------------------------+------------------------------------------------
 I use xapian-core-1.0.7 with the corresponding perl bindings. I run a 1
 writer/N reader setup, and I do reopen() a database-handle before each
 query. Nevertheless I casually get  !DatabaseModifiedErrors.

 This is what I found out so far:

 * The errors occurs after explicit flushing my most frequented index. The
 error does less often occur, if I do a sleep(1) after each explicit
 flush() before applying no changes (without flush) to the index, and it
 never occured so far with a sleep(4). This is my workaround.

 * I already set XAPIAN_FLUSH_THRESHOLD to a large value (100000).

 * I patched the xapian-core lib to log all calls of
 FlintDatabase::set_revision_number(), and the throw-points of the
 !XapianModifiedErrors, which turned out that the exception gets thrown in
 FlintTable::set_overwritten().

 * I patched again to get the caller and found out that set_overwritten()
 got called by FlintTable::block_to_cursor(), which I patched again to
 expose the condions:

 {{{
 if (REVISION(p) > REVISION(C_[j + 1].p)) {
  fprintf(stderr, "set_overwritten: from block_to_cursor() %d > %d\n",
 REVISION(p), REVISION(C_[j + 1].p));
  set_overwritten();
  return;
 }
 }}}

 and it turned out:
 {{{
 set_overwritten: from block_to_cursor() 10194 > 10192
 terminate called after throwing an instance of
 'Xapian::!DatabaseModifiedError'
 (...)
 set_overwritten: from block_to_cursor() 10195 > 10193
 terminate called after throwing an instance of
 'Xapian::!DatabaseModifiedError' set_overwritten: from block_to_cursor()
 10195 > 10193
 terminate called after throwing an instance of
 'Xapian::!DatabaseModifiedError'
 (...)
 set_overwritten: from block_to_cursor 10199 > 10197
 terminate called after throwing an instance of
 'Xapian::!DatabaseModifiedError'
 set_overwritten: from block_to_cursor 10199 > 10197
 terminate called after throwing an instance of
 'Xapian::!DatabaseModifiedError'
 }}}
 I originally tested this with xapian-1.0.6, but it also occurs in 1.0.7.

 I run xapian on Ubuntu Linux 8.04 (Hardy) with a 2.6.24-19-server kernel
 and an ext 3 file filesystem.
 The machine is an IBM x3650 with 40 GB RAM, and a ServeRAID-8k Controller
 running a Raid 10 over 6 SAS-Disks.

 My most frequented index (the one that drops the exceptions) contains
 about 850.000 documents, needs 11 Gb of disk space, gets 5-15 updates per
 second, and about 20-25  search hits per second. I flush() this index
 every 10 minutes (which takes about 60-100 seconds + 4 seconds workaround
 delay ;-)

-- 
Ticket URL: <http://trac.xapian.org/ticket/284>
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