LI-COR LI-6400, Руководство пользователя

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The LPL File System
Defragmentation
Using the LI-6400 10-5
Defragmentation
Flash memory has the advantage of being very stable and safe, even when no
power is applied to it. The disadvantage is that it cannot be overwritten (re-
used) in small pieces. This has serious implications for a file system. Every
time something is written to disk (flash memory), it must be written to a new
(unused) location. This can lead to some surprises:
For example, consider the following example. You collect a number of data
files, and find that the disk is 80% full. In an effort to make room, you delete
some no-longer-needed files, and find that the disk is now 81% full. What
happened? The reason that the space used by the files has not been reclaimed
is that it canÕt be re-used until all of it is erased. Deleting files simply marked
those files as unused, but the directory information was updated, and rewrit-
ing the directory used up some new disk space.
Eventually, of course, the disk will be come full, and something needs to be
done. The normal remedy for a full disk is to defragment it. During this pro-
cess (which is not automatic - you must initiate it), the contents of the disk are
copied to another disk, but only the portions that are still necessary
(Figure 10-3). Deleted files, for example, are not copied.
File1
File 2
File 3a
Dir
File 3b
Unused
Dir Dir
Used
ÒDeletedÓ
ÒDeletedÓ
File4 File1
File 2
File 3
Dir
Unused Used
a)
b)
Figure 10-3. Illustration of defragmentation. a) Before defragmentation, there are 3 files and 1 directory, along
with a deleted file and old directory information. b) After defragmentation, the deleted files and directories are
gone, and their space is reclaimed. Also, fragmented files (such as File3) are consolidated.