How does RECORDLENGTH affect your exp speed?

19905 ワード

c unix exp Speed recordlength bufsiz

expエクスポートツールのいくつかの一般的なパラメータはきっとみんなよく知っているに違いない.エクスポートジョブを高速化するためにdirect=yを追加する場合があります.さらに、RECORDLENGTHパラメータを設定する可能性があります.Oracle公式のこのパラメータの定義は、length of IO recordです.この解釈はあまりにも簡単で、たまにRECORDLENGTHの動作原理を探究する暇がある.

[maclean@rh2 test]$ exp help=y

Export: Release 10.2.0.4.0 - Production on Mon Nov 8 17:26:34 2010
Copyright (c) 1982, 2007, Oracle.  All rights reserved.
You can let Export prompt you for parameters by entering the EXP
command followed by your username/password:

     Example: EXP SCOTT/TIGER

Or, you can control how Export runs by entering the EXP command followed
by various arguments. To specify parameters, you use keywords:

     Format:  EXP KEYWORD=value or KEYWORD=(value1,value2,...,valueN)
     Example: EXP SCOTT/TIGER GRANTS=Y TABLES=(EMP,DEPT,MGR)
               or TABLES=(T1:P1,T1:P2), if T1 is partitioned table

USERID must be the first parameter on the command line.

Keyword    Description (Default)      Keyword      Description (Default)
--------------------------------------------------------------------------
USERID     username/password          FULL         export entire file (N)
BUFFER     size of data buffer        OWNER        list of owner usernames
FILE       output files (EXPDAT.DMP)  TABLES       list of table names
COMPRESS   import into one extent (Y) RECORDLENGTH length of IO record
GRANTS     export grants (Y)          INCTYPE      incremental export type
INDEXES    export indexes (Y)         RECORD       track incr. export (Y)
DIRECT     direct path (N)            TRIGGERS     export triggers (Y)
LOG        log file of screen output  STATISTICS   analyze objects (ESTIMATE)
ROWS       export data rows (Y)       PARFILE      parameter filename
CONSISTENT cross-table consistency(N) CONSTRAINTS  export constraints (Y)

OBJECT_CONSISTENT    transaction set to read only during object export (N)
FEEDBACK             display progress every x rows (0)
FILESIZE             maximum size of each dump file
FLASHBACK_SCN        SCN used to set session snapshot back to
FLASHBACK_TIME       time used to get the SCN closest to the specified time
QUERY                select clause used to export a subset of a table
RESUMABLE            suspend when a space related error is encountered(N)
RESUMABLE_NAME       text string used to identify resumable statement
RESUMABLE_TIMEOUT    wait time for RESUMABLE
TTS_FULL_CHECK       perform full or partial dependency check for TTS
VOLSIZE              number of bytes to write to each tape volume
TABLESPACES          list of tablespaces to export
TRANSPORT_TABLESPACE export transportable tablespace metadata (N)
TEMPLATE             template name which invokes iAS mode export

Export terminated successfully without warnings.

/*     direct=y    ，   recordlength     strace       */

[maclean@rh2 test]$ strace -o exp1.trace exp maclean/maclean file=tv.dmp tables=tv direct=y

Export: Release 10.2.0.4.0 - Production on Mon Nov 8 17:27:23 2010
Copyright (c) 1982, 2007, Oracle.  All rights reserved.
Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Export done in US7ASCII character set and UTF8 NCHAR character set
server uses ZHS16GBK character set (possible charset conversion)

About to export specified tables via Direct Path ...
. . exporting table                             TV    6602624 rows exported
Export terminated successfully without warnings.

[maclean@rh2 test]$ ps -ef|grep exp
maclean  12265 12198 55 17:27 pts/2    00:00:02 strace -o exp1.trace exp maclean/maclean file=tv.dmp tables=tv direct=y
maclean  12266 12265 24 17:27 pts/2    00:00:01 exp                 file=tv.dmp tables=tv direct=y
maclean  12269 12233  0 17:27 pts/3    00:00:00 grep exp
[maclean@rh2 test]$ pmap -x 12266
12266:   exp                 file=tv.dmp tables=tv direct=y
Address           Kbytes     RSS   Dirty Mode   Mapping
0000000000400000     608     280       0 r-x--  exp
0000000000698000      16      16       8 rwx--  exp
000000000069c000      40      40      40 rwx--    [ anon ]
000000000f330000     876     656     656 rwx--    [ anon ]
00000039f1800000     112     100       0 r-x--  ld-2.5.so
00000039f1a1b000       4       4       4 r-x--  ld-2.5.so
00000039f1a1c000       4       4       4 rwx--  ld-2.5.so
00000039f1c00000    1336     452       0 r-x--  libc-2.5.so
00000039f1d4e000    2044       0       0 -----  libc-2.5.so
00000039f1f4d000      16      16       8 r-x--  libc-2.5.so
00000039f1f51000       4       4       4 rwx--  libc-2.5.so
00000039f1f52000      20      20      20 rwx--    [ anon ]
00000039f2000000     520      20       0 r-x--  libm-2.5.so
00000039f2082000    2044       0       0 -----  libm-2.5.so
00000039f2281000       4       4       4 r-x--  libm-2.5.so
00000039f2282000       4       4       4 rwx--  libm-2.5.so
00000039f2400000       8       8       0 r-x--  libdl-2.5.so
00000039f2402000    2048       0       0 -----  libdl-2.5.so
00000039f2602000       4       4       4 r-x--  libdl-2.5.so
00000039f2603000       4       4       4 rwx--  libdl-2.5.so
00000039f2800000      88      60       0 r-x--  libpthread-2.5.so
00000039f2816000    2044       0       0 -----  libpthread-2.5.so
00000039f2a15000       4       4       4 r-x--  libpthread-2.5.so
00000039f2a16000       4       4       4 rwx--  libpthread-2.5.so
00000039f2a17000      16       4       4 rwx--    [ anon ]
00000039f5c00000      84      24       0 r-x--  libnsl-2.5.so
00000039f5c15000    2044       0       0 -----  libnsl-2.5.so
00000039f5e14000       4       4       4 r-x--  libnsl-2.5.so
00000039f5e15000       4       4       4 rwx--  libnsl-2.5.so
00000039f5e16000       8       0       0 rwx--    [ anon ]
00002b6ebde28000       8       8       8 rwx--    [ anon ]
00002b6ebde2a000   18632    5024       4 r-x--  libclntsh.so.10.1
00002b6ebf05c000    2044       0       0 -----  libclntsh.so.10.1
00002b6ebf25b000     656     656     640 rwx--  libclntsh.so.10.1
00002b6ebf2ff000     112      32      32 rwx--    [ anon ]
00002b6ebf31b000    3004    1280     416 r-x--  libnnz10.so
00002b6ebf60a000    1020       0       0 -----  libnnz10.so
00002b6ebf709000     708     256     256 rwx--  libnnz10.so
00002b6ebf7ba000       4       4       4 rwx--    [ anon ]
00002b6ebf7d8000     432     164     164 rwx--    [ anon ]
00002b6ebf844000     400       8       0 r-x--  timezlrg.dat
00002b6ebf8c5000      40      28       0 r-x--  libnss_files-2.5.so
00002b6ebf8cf000    2044       0       0 -----  libnss_files-2.5.so
00002b6ebface000       4       4       4 r-x--  libnss_files-2.5.so
00002b6ebfacf000       4       4       4 rwx--  libnss_files-2.5.so
00002b6ebfad0000       4       4       4 rwx--    [ anon ]
00007ffff2cb3000      84      64      64 rwx--    [ stack ]
ffffffffff600000    8192       0       0 -----    [ anon ]
----------------  ------  ------  ------
total kB           51408    9276    2380
[maclean@rh2 test]$ cd /proc/12266/fd
[maclean@rh2 fd]$ ls -l
   0
lrwx------ 1 maclean oinstall 64 11-08 17:27 0 -> /dev/pts/2
lrwx------ 1 maclean oinstall 64 11-08 17:27 1 -> /dev/pts/2
lrwx------ 1 maclean oinstall 64 11-08 17:27 2 -> /dev/pts/2
lr-x------ 1 maclean oinstall 64 11-08 17:27 3 -> /s01/10gdb/rdbms/mesg/expus.msb
lr-x------ 1 maclean oinstall 64 11-08 17:27 4 -> /s01/10gdb/oracore/mesg/lrmus.msb
lr-x------ 1 maclean oinstall 64 11-08 17:27 5 -> /s01/10gdb/rdbms/mesg/ocius.msb
lrwx------ 1 maclean oinstall 64 11-08 17:27 6 -> socket:[56016]
l-wx------ 1 maclean oinstall 64 11-08 17:27 7 -> /home/maclean/test/tv.dmp
l-wx------ 1 maclean oinstall 64 11-08 17:27 8 -> pipe:[56017]
lr-x------ 1 maclean oinstall 64 11-08 17:27 9 -> pipe:[56018]

/*      exp               ，       fd=>7       tv.dmp */

  trace    :
read(9, "\7\333\0\0\6\0\0\0\0\0\10\0 \0\0009:57\5\0VALID\1\0N\1\0N"..., 2064) = 2064
read(9, "2008-03-12:00:39:58\5\0VALID\1\0N\1\0N"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0LID\1\0N\1\0N\1\0N\3\0SYS\v\0I_S"..., 2064) = 2064
read(9, "\376\377\3\0\302\5$\3\0\302\5$\5\0INDEX\7\0xl\3\f\1(<\7\0xl"..., 1958) = 1958
read(9, "\0\327\0\0\6\0\0\0\0\00059\5\0VALID\1\0N\1\0N\1\0N\5\0OU"..., 2064) = 215
write(7, "9:57\5\0VALID\1\0N\1\0N\1\0N\3\0SYS\t\0ATEMP"..., 4096) = 4096
write(7, "\0N\1\0N\3\0SYS\v\0I_SUMDEP$_2\376\377\3\0\302\5\17\3\0"..., 4096) = 4096
write(8, "\0r\0\0\6\0\0\0\0\0\3[\235@\177\202\277n+\0\0\0 \0\0\0\0\0\0\200a\314"..., 114) = 114
read(9, "\7\333\0\0\6\0\0\0\0\0\10\0 \0\0ALID\1\0N\1\0N\1\0N\5\0OU"..., 2064) = 2064
read(9, "\0N\1\0N\1\0N\3\0SYS\t\0MON_MODS$\376\377\3\0\302\5T\3"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\1)\1\7\0xl\3\f\1)\1\23\0002008-03-"..., 2064) = 2064
read(9, "ABLE\7\0xl\3\f\1)\26\7\0xl\3\f\1)\26\23\0002008-03-"..., 1958) = 1958
read(9, "\0\327\0\0\6\0\0\0\0\0\302\0061\3\0\302\0061\5\0INDEX\7\0xl\3\f\1"..., 2064) = 215
write(7, "ALID\1\0N\1\0N\1\0N\5\0OUTLN\17\0OL$NODE_OL"..., 4096) = 4096
write(7, ")\1\7\0xl\3\f\1)\1\23\0002008-03-12:00:40:00"..., 4096) = 4096
write(8, "\0r\0\0\6\0\0\0\0\0\3[\236@\177\202\277n+\0\0\0 \0\0\0\0\0\0\200a\314"..., 114) = 114

/*       9        ，          TV       ;
   fd 8    system call     ，        */

[maclean@rh2 test]$ strace -o exp2.trace exp maclean/maclean file=tv.dmp tables=tv direct=y recordlength=32768

Export: Release 10.2.0.4.0 - Production on Mon Nov 8 18:31:07 2010

Copyright (c) 1982, 2007, Oracle.  All rights reserved.

Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Export done in US7ASCII character set and UTF8 NCHAR character set
server uses ZHS16GBK character set (possible charset conversion)

About to export specified tables via Direct Path ...
. . exporting table                             TV    6602624 rows exported
Export terminated successfully without warnings.

   recordlength 32k  system call trace:

read(9, "\7\333\0\0\6\0\0\0\0\0\10\0\200\0\0ONYM\7\0xl\3\f\2\t9\7\0xl"..., 2064) = 2064
read(9, "\5\0VALID\1\0N\1\0N\1\0N\7\0OLAPSYS\10\0CWM$U"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0002:01:09:00\5\0VALID\1\0N\1\0"..., 2064) = 2064
read(9, "\0N\7\0OLAPSYS\26\0CWM2$AWDIMCREATEACC"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\0\303\5T\2\5\0TABLE\7\0xl\3\f\2
\4\7"..., 2064) = 2064
read(9, "\5T\25\4\0\303\5T\25\5\0INDEX\7\0xl\3\f\2
\5\7\0xl\3\f\2"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\4\0\303\5T'\5\0INDEX\7\0xl\3\f\2
\5"..., 2064) = 2064
read(9, "ID\1\0N\1\0N\1\0N\7\0OLAPSYS\23\0CWM2$HIERA"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0007:32\5\0VALID\1\0N\1\0N\1\0N\7\0"..., 2064) = 2064
read(9, "-10-20:20:07:33\5\0VALID\1\0N\1\0N\1\0N\7"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\0VALID\1\0N\1\0N\1\0N\7\0OLAPS"..., 2064) = 2064
read(9, "\0CWM2_OLAP_MEASURE\376\377\4\0\303\5U\"\376\377\7\0PA"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\0N\1\0N\1\0N\6\0PUBLIC\23\0CWM2"..., 2064) = 2064
read(9, "008-03-12:01:09:11\5\0VALID\1\0N\1\0N\1"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\0N\7\0OLAPSYS\35\0ALL$OLAP2"..., 2064) = 2064
read(9, "TTR_USES\376\377\4\0\303\5V\v\376\377\4\0VIEW\7\0xl\3\f\2
"..., 1958) = 1958
read(9, "\3\v\0\0\6\0\0\0\0\0\2
\23\7\0xn
\24\25\10'\23\0002008-03-"..., 2064) = 779
write(7, "ONYM\7\0xl\3\f\2\t9\7\0xl\3\f\2\t9\23\0002008-03-"..., 4096) = 4096
write(7, "008-03-12:01:09:02\5\0VALID\1\0N\1\0N\1"..., 28672) = 28672

/*       exp       (accumulates)   32k         ，   2 write      4096 28672 bytes
       ，      32k            ，          8k    8k
*/

/*   pmap                 */

00002b9f6e034000     708     256     256 rwx--  libnnz10.so
00002b9f6e0e5000       4       4       4 rwx--    [ anon ]
00002b9f6e103000     432     212     212 rwx--    [ anon ]
00002b9f6e16f000     400       8       0 r-x--  timezlrg.dat
00002b9f6e1f0000      40      28       0 r-x--  libnss_files-2.5.so

/*      exp      anon(00002b9f6e103000     432     212     212 rwx--    [ anon ])  rss Dirty,
        164k    212k，   48k
*/

[maclean@rh2 test]$ strace -o exp3.trace exp maclean/maclean file=tv.dmp tables=tv direct=y recordlength=65535

Export: Release 10.2.0.4.0 - Production on Mon Nov 8 17:53:25 2010
Copyright (c) 1982, 2007, Oracle.  All rights reserved.
Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Export done in US7ASCII character set and UTF8 NCHAR character set
server uses ZHS16GBK character set (possible charset conversion)

About to export specified tables via Direct Path ...
. . exporting table                             TV    6602624 rows exported
Export terminated successfully without warnings.

/*  65535 recordlength   strace    */

read(9, "\7\333\0\0\6\0\0\0\0\0\10\377\377\0\0MADMIN\21\0SYS_IOT_T"..., 2064) = 2011
read(9, "\7\333\0\0\6\0\0\0\0\00010-09-29:18:14:08\5\0VAL"..., 2064) = 2064
read(9, "4:09\5\0VALID\1\0N\1\0N\1\0N\3\0SYS\23\0AQ$AQ"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\4\0\303\6 b\5\0INDEX\7\0xn\t\36\17!0"..., 2064) = 2064
read(9, ":21\5\0VALID\1\0N\1\0N\1\0N\3\0SYS\4\0HR11\376\377"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0J$\4\0P101\4\0\303\6!>\4\0\303\6\"9\17\0"..., 2064) = 2064
read(9, "P101\4\0\303\6!O\4\0\303\6\">\17\0INDEX PARTITIO"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0I1CCOL$\4\0P101\4\0\303\6!a\4\0\303"..., 2064) = 2064
read(9, ":44\5\0VALID\1\0N\1\0N\1\0N\3\0SYS\16\0WRH$_P"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0N_HISTORY_PK\31\0WRH$_ACT"..., 2064) = 2064
read(9, "SQLSTAT_PK\31\0WRH$_SQLSTA_14284528"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\0002010-10-18:20:48:28\5\0"..., 2064) = 2064
read(9, ";\f\23\0002010-10-20:19:58:11\5\0VALID\1\0"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0RT$\376\377\4\0\303\6$W\376\377\5\0TABLE\7\0"..., 2064) = 2064
read(9, "38C00004$$\376\377\4\0\303\6%(\4\0\303\6%(\3\0LOB\7\0x"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\6%G\376\377\5\0TABLE\7\0xn
\24\25\10\34\7"..., 2064) = 2064
read(9, "VALID\1\0N\1\0N\1\0N\3\0SYS\30\0GV_OLAPI_SE"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\1\0N\1\0N\3\0SYS\31\0OLAPI_MEM"..., 2064) = 2064
read(9, "n
\24\25\t
\7\0xn
\24\25\t
\23\0002010-10-20:20:0"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\4\0TYPE\7\0xn
\24\25\t\v\7\0xn
\24\25"..., 2064) = 2064
read(9, "\16\23\0002010-10-20:20:08:12\5\0VALID\1\0N"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\7\0xn
\24\25\t\25\23\0002010-10-20:"..., 2064) = 2064
read(9, "ITIONSTRUCT\376\377\4\0\303\6(\4\376\377\4\0TYPE\7\0xn
"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\4\0VIEW\7\0xn
\24\25\t\27\7\0xn
\24\25"..., 2064) = 2064
read(9, " PARTITION\7\0xn\v\7\0238\10\7\0xn\v\7\0238\10\23\00020"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0-11-07:18:55:08\5\0VALID"..., 2064) = 2064
read(9, "ITION\7\0xn\v\7\0238
\7\0xn\v\7\0238
\23\0002010-11"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\000010-11-07:22:52:09\5\0VA"..., 2064) = 2064
read(9, "-11-07:22:52:10\5\0VALID\1\0N\1\0N\1\0N\6"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0T_SOO_BUFFER_BUSY\376\377\4\0\303"..., 2064) = 2064
read(9, "\0xn
\36\27\5\33\7\0xn
\37\27\6\5\23\0002010-10-23:01"..., 1958) = 1958
read(9, "\7\333\0\0\6\0\0\0\0\0\6+ \17\0TABLE PARTITION\7\0"..., 2064) = 2064
read(9, "09\5\0VALID\1\0N\1\0N\1\0N\3\0SYS\27\0WRH$_DB"..., 1477) = 1477
write(7, "0-09-29:17:56:12\5\0VALID\1\0N\1\0N\1\0N"..., 4096) = 4096
write(7, "05465\376\377\4\0\303\6\37\\\4\0\303\6\37\\\5\0INDEX\7\0xn\t\35"..., 61440) = 61440

/*   exp    (accumulates)   65536 bytes       ，   2 write            
          recordlength  direct patch  buffer size
*/

ではrecordlengthはexpの効率にどのように影響しますか?以上の説明から、directダイレクトパス読み取り方式を使用する場合、recordlengthは読み取り(read)に影響しません.どのようなrecordlengthを設定しても、expプロセスに必要なreadの総作業量は同じです.違いは、writeの回数と1回で書かれたデータ量であり、recordlengthを設定しない例では、8192バイトのデータを2回書くwriteが必要であり、dumpファイルが600 MBの場合は153600回書く必要がある.一方、recordlengthが65535に設定されている場合、その書き出し(4096+61440)=65536も2回write callを行うだけで、dumpファイルが600 MBの場合は19200回しか書く必要はありません.より大きなrecordlengthを使用すると、expジョブに要するCPU時間と実際のIO回数を効果的に低減し、導出速度を極めて高速化できることが分かる.IO層を表現すると,ユーザはiostatなどのツールを用いて書き出し段階での書き出しIOの大幅な上昇を表示可能にする.recordlengthを使用する直接的な代価は、expプロセスがより多くのメモリ(一般的に100~200 k程度)を消費することであり、現在の目から見れば、このメモリは重要ではない.recordlengthには65535の最大値(maximum)が存在し、65536を超えるrecordlengthを設定すると、expプログラムは、Note:RECORDLENGTH=65536 truncated to 65535のような情報を返します.初期バージョン(734 or 8 i)ではこの制限は存在せず、expの開発者が65535より大きいrecordlengthがエクスポートを加速させるのに役立たないことを徐々に認識していることを示している.
Metalinkドキュメント[ID 134966.1]では、recordlengthのデフォルト値をどのように判断するかを示します.
Problem Description ------------------- You want to export the entire database and move it to a different platform. The RECORDLENGTH parameter must be specified during the export since each platform has a different default. How do you determine the default RECORDLENGTH value for each platform? Solution Description -------------------- Look for the BUFSIZ-Parameter in the File/usr/include/stdio.h file to determine the default size of the output record length. If no value is specified for the BUFSIZ parameter then a default of 512 bytes will be used. Explanation ----------- If the RECORDLENGTH parameter is not explicitly set on EXP/IMP, ORACLE will use the value specified for BUFSIZ in the/usr/include/stdio.h file to determine the size of it's output record length.
The BUFFER parameter applies ONLY to conventional path Export. It has no effect on a direct path Export. This
BUFFER parameter specifies the size (in bytes) of the buffer used to fetch rows. It determines the maximum number of rows in an array, fetched by Export. For direct path Export, use the RECORDLENGTH parameter to specify the size of the buffer that Export uses for writing to the export file.
The RECORDLENGTH parameter specifies the length (in bytes) of the file record. You can use this parameter to specify the size of the Export I/O buffer (highest value is 64 kb). Changing the RECORDLENGTH parameter affects only the size of data that accumulates before writing to disk. It does not affect the operating system file block size. If you do not define this parameter, it defaults to your platform-dependent value for BUFSIZ (1024 bytes in most cases).
BUFSIZ自体はUNIX/LINEUXシステムでI/O buffersizeを制御するパラメータであり、興味のある研究があれば有名なapue書(W.Richard Stevensのを参考にすることができ、この本は誰のUnix C啓蒙書にもなる.
The value of BUFSIZ is chosen on each system so as to make stream I/O efficient. So it is a good idea to use BUFSIZ as the size for the buffer when you call setvbuf. Actually, you can get an even better value to use for the buffer size by means of the fstat system call: it is found in the st_blksize field of the file attributes.Sometimes people also use BUFSIZ as the allocation size of buffers used for related purposes, such as strings used to receive a line of input with fgets (see section 12.8 Character Input). There is no particular reason to use BUFSIZ for this instead of any other integer, except that it might lead to doing I/O in chunks of an efficient size.

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