Linux²»ÐèÒª´ÅÅÌËéƬÕûÀí

Linux²»ÐèÒª´ÅÅÌËéƬÕûÀí¡£
ÒÔÏÂÒý×Ôlinux¹Ù·½ÍøÕ¾¶ÔËéƬµÄ½â˵£ºÀ´Ô´ÓÚ
http://www.linux.org/docs/ldp/howto/Partition/appendix.html#fragmentation

ÒýÓÃ:
10.4. Some facts about file systems and fragmentation
Disk
space is administered by the operating system in units of blocks and
fragments of blocks. In ext2, fragments and blocks have to be of the
same size, so we can limit our discussion to blocks.
Files
come in any size. They don't end on block boundaries. So with every
file a part of the last block of every file is wasted. Assuming that
file sizes are random, there is approximately a half block of waste for
each file on your disk. Tanenbaum calls this "internal fragmentation"
in his book "Operating Systems".
You can guess the number of files on your disk by the number of allocated inodes on a disk. On my disk
# df -i
Filesystem Inodes IUsed IFree %IUsed Mounted on
/dev/hda3 64256 12234 52022 19% /
/dev/hda5 96000 43058 52942 45% /var
there
are about 12000 files on / and about 44000 files on /var. At a block
size of 1 KB, about 6+22 = 28 MB of disk space are lost in the tail
blocks of files. Had I chosen a block size of 4 KB, I had lost 4 times
this space.
Data transfer is faster for large contiguous
chunks of data, though. That's why ext2 tries to preallocate space in
units of 8 contigous blocks for growing files. Unused preallocation is
released when the file is closed, so no space is wasted.
Noncontiguous
placement of blocks in a file is bad for performance, since files are
often accessed in a sequential manner. It forces the operating system
to split a disk access and the disk to move the head. This is called
"external fragmentation" or simply "fragmentation" and is a common
problem with MS-DOS file systems. In conjunction with the abysmal
buffer cache used by MS-DOS, the effects of file fragmentation on
performance are very noticeable. DOS users are ac