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The initialization parameter LOG_BUFFER determines the size of Redo Log Buffer. Although the size of Redo Entries in LOG_BUFFER is in units of bytes, LGWR still writes Redo to disk in units of Block. Redo Block Size is fixed in Oracle source code and is related to the operating system.

Most operating systems are based on 512bytes, such as Solaris, AIX, Windows NT/2000, Linux and so on. This Log Size can be obtained from the internal view of Oracle:

Sys@TQGZS > select max (lebsz) from x$kccle

MAX (LEBSZ)

-

five hundred and twelve

Additional information can also be roughly calculated from the statistics in v$sysstat, such as Redo Size (size of Redo information), Redo Wastage (size of wasted Redo), Redo Blocks Written (number of Redo Block written by LGWR), etc. Each Redo Block Header takes up 16 Bytes, so the Redo Block Size can be roughly calculated as follows:

Sys@TQGZS > select name,value from v$sysstat

2 where name in ('redo size','redo wastage','redo blocks written')

NAME VALUE

Redo size 13253016

Redo wastage 225776

Redo blocks written 27416

Sys@TQGZS > select ceil (16 + (132530160225776) / 27416) rbsize from dual

RBSIZE

-

five hundred and eight

Under Linux/UNIX, Oracle also provides another command-line tool that can be used to check the Block Size size of files:

[oracle@tqgzs: ~ / oradata/tqgzs] $dbfsize redo01.log

Database file: redo01.log

Database file type: file system

Database file size: 102400 512 byte blocks

[oracle@tqgzs: ~ / oradata/tqgzs] $dbfsize system01.dbf

Database file: system01.dbf

Database file type: file system

Database file size: 89600 8192 byte blocks

[oracle@tqgzs: ~ / oradata/tqgzs] $which dbfsize

~ / product/10.2.0/db_1/bin/dbfsize

As you can see from the output above, the Block Size of the log file is 512 Bytes, while the Block Size of the data file is 8192 Bytes. Of course, you can also obtain the log file block size by dumping the log file. The dump log file header can be named as follows:

Alter session set events' immediate trace name redohdr level 10'

If you look at the trace file, you can get information like the following. The bsz in the output means that Redo Block Size is 512Bytes:

LOG FILE # 1:

(name # 3) / u01/oracle/oradata/tqgzs/redo01.log

Thread 1 redo log links: forward: 2 backward: 0

Siz: 0x19000 seq: 0x00000077 hws: 0x4 bsz: 512 nab: 0x2 flg: 0x1 dup: 1

Archive links: fwrd: 0 back: 0 Prev scn: 0x0000.005f5e8e

Low scn: 0x0000.005f5e90 12/13/2009 23:33:58

Next scn: 0x0000.005f5e92 12/13/2009 23:34:03

Sometimes when an archiving error occurs in the database, a prompt is given to inform blocksize=512.

ORA-19502: write error on file "/ data1/oraarc/PTL017151.arc", blockno 169985 (blocksize=512)

ORA-27072: skgfdisp: I/O error

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