Time and space optimization

If you are using aufsfilesystem, the system attempts to optimize the
[url=javascript:;]time[/url]
it takes to perform operations on the filesystem. In certain circumstances, the system attempts to optimize the
[url=javascript:;]space[/url]
on the storage media being used by the filesystem.
Space optimization occurs when file fragmentation exceeds:(free - 2) %of free disk space. When file fragmentation exceeds this value, this message is displayed:Optimization changed from TIME to and the system will begin to optimize the space used by the filesystem.
The value offreeis, by default, 10%, but it can be set to any value using the
mkfs(1M)
command. The main use offreeis to specify the minimum percentage of free disk space allowed. It is also used to limit the fragmentation of the filesystem.
When a file grows past an 8K boundary under time optimization, the system allocates a fragment (which is 1K in size) to contain the data. When the boundary of the fragment is exceeded, the system allocates a new 8K block, and the fragment is copied to it. This means that there could be nearly 7K of wasted space at the end of some files.
With space optimization, however, whenever a file grows past a 1K boundary, all the fragments since the last 8K boundary are copied into a new, larger fragment.
If your system has swapped to space optimization, and you want to revert to time optimization, you must increase the value offree. Use
tunefs(1M)
to do this. You can increasefreeto any value you want; however, keep in mind the two functions of thefreeparameter if you want to set realistic values.freeis used to do the following:

• Set the minimum percentage of free disk space allowed. Therefore, if you set the value offreetoo high, ordinary (nonprivileged) users will not be able to write files (as described in
  ``Creating a filesystem with mkfs''
  ).
  
• Set the maximum percentage of free disk space that can be fragments. Therefore, if you set the value offreetoo low, the optimization will change from time to space and file writes will be slower.

Set the value offreecarefully, taking into account the typical file size and the number of directories on your system.

本文来自ChinaUnix博客，如果查看原文请点：http://blog.chinaunix.net/u/4206/showart_1385732.html

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