IT16125: INSTANCE CAN GET BLOCKED IN CHECKPOINT IF MAX_FILL_DATA_PAGES SET TO 1 AND CONCURRENT INSERTS OF 'LONG' ROWS OCCURS

APAR status

• Closed as program error.

Error description

• If you have the onconfig parameter MAX_FILL_DATA_PAGES set to 1,
  Informix Server tries to fill the data pages in tables
  containing VARCHAR columns to their maximum. The instance uses
  so called 'page free space cache' (PFSC) which holds information
  about recently used data pages and an amount of free space on
  them. When a session inserts a new row, it looks into PFSC at
  first and if no page with enough space for the new row is found
  there, the session searches the bitmap pages in the table,
  looking for pages marked as semifull (0x4). If some is found,
  the session reads the page and tries to insert the new row. If
  the free space on that page is not enough, the session continues
  in searching the bitmaps and trying the next semifull data page.
  This goes until the a page with enough free space is found and
  the row inserted. This whole process is done as a part of one
  critical section.
  In case all or majority of the pages in the table are marked as
  semifull, but the free space on them is not enough for the new
  row, the session can potentially read the whole table before it
  finds the proper data page. In case there are more sessions
  doing the same type of 'long row' insert into the same table at
  the same time and they enter their critical section just before
  a checkpoint occurs, the instance can get blocked in the
  checkpoint for very long time (possibly up to tens of minutes;
  depends on the configuration and performance of your system).
  The symptoms are:
  
  - 'onstat -u' output shows several sessions in critical section,
  all but one waiting on the same buffer:
  
  Userthreads
  address          flags   sessid   user     tty      wait
  tout locks nreads   nwrites
  7000000805412c8  ---PX-- 21       informix 3        0
  0    2     42       0
  700000080542338  B--PX-- 22       informix 6
  700000012c377b0  0    2     0        0
  700000080543be0  B--PX-- 25       informix 6
  700000012c377b0  0    2     0        0
  700000080544418  B--PX-- 24       informix 6
  700000012c377b0  0    2     0        0
  700000080544c50  B--PX-- 23       informix 6
  700000012c377b0  0    2     0        0
  700000080545488  B--PX-- 26       informix 6
  700000012c377b0  0    2     0        0
  700000080545cc0  B--PX-- 27       informix 6
  700000012c377b0  0    2     0        0
  7000000805464f8  B--PX-- 28       informix 6
  700000012c377b0  0    2     0        0
  700000080546d30  B--PX-- 29       informix 6
  700000012c377b0  0    2     0        0
  700000080547568  B--PX-- 30       informix 6
  700000012c377b0  0    2     0        0
  700000080547da0  B--PX-- 31       informix 6
  700000012c377b0  0    2     0        0
  7000000805485d8  B--PX-- 32       informix 6
  700000012c377b0  0    2     0        0
  700000080548e10  B--PX-- 33       informix 6
  700000012c377b0  0    2     0        0
  700000080549648  B--PX-- 34       informix 6
  700000012c377b0  0    2     0        0
  700000080549e80  B--PX-- 35       informix 6
  700000012c377b0  0    2     0        0
  70000008054a6b8  B--PX-- 36       informix 6
  700000012c377b0  0    2     0        0
  70000008054aef0  B--PX-- 37       informix 6
  700000012c377b0  0    2     0        0
  ...
  
  - 'onstat -X' shows that the buffer everyone is waiting on holds
  a bitmap page (pgflgs 0x804)
  
  Buffers (Access)
  address          owner            flags pagenum          memaddr
  nslots pgflgs scount   waiter
  700000012c377b0  7000000805412c8  80    3:1023
  70000001f6ea000  0      804    0        700000080542338
  
  700000080544418
  
  700000080545488
  
  700000080546d30
  
  ....
  
  
  - 'onstat -g stk' for the main_loop thread shows it's in
  wait4critex() call
  - 'onstat -g ppf <partnum>' for the table into which the
  sessions are inserting will show a huge and ever growing numbers
  in bfrd and bfwrt
  
  Partition profiles
  partnum    lkrqs lkwts dlks  touts isrd  iswrt isrwt isdel bfrd
  bfwrt    seqsc rhitratio
  0x300043   90    0     0     0     0     31    0     0
  26403161 24799999 0     94
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED:                                              *
  * Any                                                          *
  ****************************************************************
  * PROBLEM DESCRIPTION:                                         *
  * See Error Description                                        *
  ****************************************************************
  * RECOMMENDATION:                                              *
  * Update to IBM Informix Server 11.50.xC10                     *
  ****************************************************************
  

Problem conclusion

• Problem Fixed In IBM Informix Server 11.50.xC10
  

Temporary fix

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

Fix information

• Fixed component name

  IBM IDS ENTRP E

• Fixed component ID

  5724L2304

Applicable component levels

• RB15 PSN

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