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# mkswap -f /dev/hda5# swapon /dev/hda5
# mount -t ext2 /dev/sda1 /opt| -t ext2 | File system type.
Other types you are likely to use are:
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| /dev/sda1 | Device name. Other device names you are likely to use:
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| /opt | mount point. This is where you want to "see" your partition.
When you type ls /opt, you can see what is in /dev/sda1.
If there are already some directories and/or files under /opt,
they will be invisible after this mount command.
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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 DOS file systems.
ext2 has several strategies to avoid external fragmentation. Normally fragmentation is not a large problem in ext2, not even on heavily used partitions such as a USENET news spool. While there is a tool for defragmentation of ext2 file systems, nobody ever uses it and it is not up to date with the current release of ext2. Use it, but do so on your own risk.
The MS-DOS file system is well known for its pathological managment of disk space. 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.
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 the 1 KB default block size if possible. You may want to experiment with a block size of 2 KB for some partitions, but expect to meet some seldom exercised bugs: Most people use the default.
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