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Sparse Files

作者：OoNiceDream【转载时请务必以超链接形式标明文章原始出处和作者信息】
链接：http://www.dbaroad.me/archives/2009/02/sparse-files.html

在Unix操作系统上，Oracle创建TEMP表空间，添加TEMP文件时，可以观察到瞬间就完成了，而且文件系统的使用率又没增长多少。这其实就是因为TEMP文件是Sparse Files的原因。

什么是Sparse Files

On many Unix filesystems, files containing long strings of nulls can be stored
much more efficiently than other files. To be specific, if a string of nulls
spans an entire allocation block, that whole block is not stored on disk at all.
Files where one or more blocks are omitted in this way are called sparse files.
The missing blocks are also known as holes.

下面来看看添加TEMP文件时的情况：

[/home/oraadm]$ bdf
Filesystem          kbytes    used   avail %used Mounted on
/dev/vg00/lvol3    2097152 1411752  680104   67% /
/dev/vg00/lvol1    1031544  116184  812200   13% /stand
/dev/vg00/lvol8    5144576 2820144 2307448   55% /var
/dev/vg00/lvol7    3145728 1286120 1845104   41% /usr
/dev/vg00/lvol4    2097152  331432 1754600   16% /tmp
/dev/vgdba/lvol1   533790720 90518648 439809136   17% /oradata
/dev/vg00/lvol6    5242880 3181736 2045160   61% /opt
/dev/vg00/lvol5    4194304 1838336 2337584   44% /home
/dev/vg00/lvol9    10256384 6144404 3983508   61% /app
[/oradata/TADBA]$ exit

SQL> set timi on
SQL> alter tablespace temp add tempfile
  2  '/oradata/TADBA/temp03.dbf' size 5120M autoextend off;

Tablespace altered.

Elapsed: 00:00:00.03
SQL> exit
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.8.0 - 64bit
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.8.0 - Production
[/oradata/TADBA]$ bdf
Filesystem          kbytes    used   avail %used Mounted on
/dev/vg00/lvol3    2097152 1411752  680104   67% /
/dev/vg00/lvol1    1031544  116184  812200   13% /stand
/dev/vg00/lvol8    5144576 2836528 2291200   55% /var
/dev/vg00/lvol7    3145728 1286120 1845104   41% /usr
/dev/vg00/lvol4    2097152  331456 1754576   16% /tmp
/dev/vgdba/lvol1   533790720 90518728 439809056   17% /oradata
/dev/vg00/lvol6    5242880 3181736 2045160   61% /opt
/dev/vg00/lvol5    4194304 1838336 2337584   44% /home
/dev/vg00/lvol9    10256384 6144492 3983424   61% /app
[/oradata/TADBA]$
[/oradata/TADBA]$ ls -l temp03.dbf
-rw-r-----   1 oracle     dba        5368717312 Feb 10 14:17 temp03.dbf
[/oradata/TADBA]$ du -k temp03.dbf
72      temp03.dbf
[/oradata/TADBA]$

添加TEMP文件后，空间使用率，没有上升，只增长了439809136-439809056=80K。
只有当TEMP文件使用后，空间才会分配，这一点在添加TEMP文件时需要引起注意。

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