Oracle Scratchpad

March 11, 2015

Flashback Logging

Filed under: Flashback,Infrastructure,Locks,Oracle,Troubleshooting — Jonathan Lewis @ 3:21 pm BST Mar 11,2015

One of the waits that is specific to ASSM (automatic segment space management) is the “enq: FB – contention” wait. You find that the “FB” enqueue has the following description and wait information when you query v$lock_type, and v$event_name:


SQL> execute print_table('select * from v$lock_type where type = ''FB''')
TYPE                          : FB
NAME                          : Format Block
ID1_TAG                       : tablespace #
ID2_TAG                       : dba
IS_USER                       : NO
DESCRIPTION                   : Ensures that only one process can format data blocks in auto segment space managed tablespaces

SQL> execute print_table('select * from v$event_name where name like ''enq: FB%''')
EVENT#                        : 806
EVENT_ID                      : 1238611814
NAME                          : enq: FB - contention
PARAMETER1                    : name|mode
PARAMETER2                    : tablespace #
PARAMETER3                    : dba
WAIT_CLASS_ID                 : 1893977003
WAIT_CLASS#                   : 0
WAIT_CLASS                    : Other

This tells us that a process will acquire the lock when it wants to format a batch of blocks in a segment in a tablespace using ASSM – and prior experience tells us that this is a batch of 16 consecutive blocks in the current extent of the segment; and when we see a wait for an FB enqueue we can assume that two session have simultaneously tried to format the same new batch of blocks and one of them is waiting for the other to complete the format. In some ways, this wait can be viewed (like the “read by other session” wait) in a positive light – if the second session weren’t waiting for the first session to complete the block format it would have to do the formatting itself, which means the end-user has a reduced response time. On the other hand the set of 16 blocks picked by a session is dependent on its process id, so the second session might have picked a different set of 16 blocks to format, which means in the elapsed time of one format call the segment could have had 32 blocks formatted – this wouldn’t have improved the end-user’s response time, but it would mean that more time would pass before another session had to spend time formatting blocks. Basically, in a highly concurrent system, there’s not a lot you can do about FB waits (unless, of course, you do some clever partitioning of the hot objects).

There is actually one set of circumstances where you can have some control of how much time is spent on the wait, but before I mention it I’d like to point out a couple more details about the event itself. First, the parameter3/id2_tag is a little misleading: you can work out which blocks are being formatted (if you really need to), but the “dba” is NOT a data block address (which you might think if you look at the name and a few values). There is a special case when the FB enqueue is being held while you format the blocks in the 64KB extents that you get from system allocated extents, and there’s probably a special case (which I haven’t bothered to examine) if you create a tablespace with uniform extents that aren’t a multiple of 16 blocks, but in the general case the “dba” consists of two parts – a base “data block address” and a single (hex) digit offset identifying which batch of 16 blocks will be formatted.

For example: a value of 0x01800242 means start at data block address 0x01800240, count forward 2 * 16 blocks then format 16 blocks from that point onwards. Since the last digit can only range from 0x0 to 0xf this means the first 7 (hex) digits of a “dba” can only reference 16 batches of 16 blocks, i.e. 256 blocks. It’s not coincidence (I assume) that a single bitmap space management block can only cover 256 blocks in a segment – the FB enqueue is tied very closely to the bitmap block.

So now it’s time to ask why this discussion of the FB enqueue appears in an article titled “Flashback Logging”. Enable the 10704 trace at level 10, along with the 10046 trace at level 8 and you’ll see. Remember that Oracle may have to log the old version of a block before modifying it and if it’s a block that’s being reused it may contribute to “physical reads for flashback new” – here’s a trace of a “format block” event:


*** 2015-03-10 12:50:35.496
ksucti: init session DID from txn DID:
ksqgtl:
        ksqlkdid: 0001-0023-00000014

*** 2015-03-10 12:50:35.496
*** ksudidTrace: ksqgtl
        ktcmydid(): 0001-0023-00000014
        ksusesdi:   0000-0000-00000000
        ksusetxn:   0001-0023-00000014
ksqgtl: RETURNS 0
WAIT #140627501114184: nam='db file sequential read' ela= 4217 file#=6 block#=736 blocks=1 obj#=192544 tim=1425991835501051
WAIT #140627501114184: nam='db file sequential read' ela= 674 file#=6 block#=737 blocks=1 obj#=192544 tim=1425991835501761
WAIT #140627501114184: nam='db file sequential read' ela= 486 file#=6 block#=738 blocks=1 obj#=192544 tim=1425991835502278
WAIT #140627501114184: nam='db file sequential read' ela= 522 file#=6 block#=739 blocks=1 obj#=192544 tim=1425991835502831
WAIT #140627501114184: nam='db file sequential read' ela= 460 file#=6 block#=740 blocks=1 obj#=192544 tim=1425991835503326
WAIT #140627501114184: nam='db file sequential read' ela= 1148 file#=6 block#=741 blocks=1 obj#=192544 tim=1425991835504506
WAIT #140627501114184: nam='db file sequential read' ela= 443 file#=6 block#=742 blocks=1 obj#=192544 tim=1425991835504990
WAIT #140627501114184: nam='db file sequential read' ela= 455 file#=6 block#=743 blocks=1 obj#=192544 tim=1425991835505477
WAIT #140627501114184: nam='db file sequential read' ela= 449 file#=6 block#=744 blocks=1 obj#=192544 tim=1425991835505985
WAIT #140627501114184: nam='db file sequential read' ela= 591 file#=6 block#=745 blocks=1 obj#=192544 tim=1425991835506615
WAIT #140627501114184: nam='db file sequential read' ela= 449 file#=6 block#=746 blocks=1 obj#=192544 tim=1425991835507157
WAIT #140627501114184: nam='db file sequential read' ela= 489 file#=6 block#=747 blocks=1 obj#=192544 tim=1425991835507684
WAIT #140627501114184: nam='db file sequential read' ela= 375 file#=6 block#=748 blocks=1 obj#=192544 tim=1425991835508101
WAIT #140627501114184: nam='db file sequential read' ela= 463 file#=6 block#=749 blocks=1 obj#=192544 tim=1425991835508619
WAIT #140627501114184: nam='db file sequential read' ela= 685 file#=6 block#=750 blocks=1 obj#=192544 tim=1425991835509400
WAIT #140627501114184: nam='db file sequential read' ela= 407 file#=6 block#=751 blocks=1 obj#=192544 tim=1425991835509841

*** 2015-03-10 12:50:35.509
ksqrcl: FB,16,18002c2
ksqrcl: returns 0

Note: we acquire the lock (ksqgtl), read 16 blocks by “db file sequential read”, write them to the flashback log (buffer), format them in memory, release the lock (ksqrcl). That lock can be held for quite a long time – in this case 13 milliseconds. Fortunately the all the single block reads after the first have been accelerated by O/S prefetching, your timings may vary.

The higher the level of concurrent activity the more likely it is that processes will collide trying to format the same 16 blocks (the lock is exclusive, so the second will request and wait, then find that the blocks are already formatted when it finally get the lock). This brings me to the special case where waits for the FB enqueue waits might have a noticeable impact … if you’re running parallel DML and Oracle decides to use “High Water Mark Brokering”, which means the parallel slaves are inserting data into a single segment instead of each using its own private segment and leaving the query co-ordinator to clean up round the edges afterwards. I think this is most likely to happen if you have a tablespace using fairly large extents and Oracle thinks you’re going to process a relatively small amount of data (e.g. small indexes on large tables) – the trade-off is between collisions between processes and wasted space from the private segments.

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