Text 4330, 72 rader
Skriven 2005-10-28 06:49:26 av David Drummond (3:640/305)
Ärende: Linux file systems and degragging
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From a Web forum:-....


7.3. 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 accustomed to
defragging their disks every few weeks and some have even developed some
ritualistic beliefs regarding defragmentation.

None of these habits should be carried over to Linux and ext2. Linux
native file systems do not need defragmentation under normal use and
this includes any condition with at least 5% of free space on a disk.
There is a defragmentation tool for ext2 called defrag, but users are
cautioned against casual use. A power outage during such an operation
can trash your file system. Since you need to back up your data anyway,
simply writing back from your copy will do the job.

The MS-DOS file system is also known to lose large amounts of disk space
due to internal fragmentation. For partitions larger than 256 MB, DOS
block sizes grow so large that they are no longer useful (This has been
corrected to some extent with FAT32). Ext2 does not force you to choose
large blocks for large file systems, except for very large file systems
in the 0.5 TB range (that's terabytes with 1 TB equaling 1024 GB) and
above, where small block sizes become inefficient. So unlike DOS there
is no need to split up large disks into multiple partitions to keep
block size down.

Use a 1Kb block size if you have many small files. For large partitions,
4Kb blocks are fine.

-- 

regards

David

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 * Origin: news://fido.jabberwocky.com.au (3:640/305)