| Linux Partition HOWTO | ||
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At the shell prompt, I begin making the file systems on my partitions. Continuing with the example in Section 5.1.3, this is:
# mke2fs /dev/sda1 |
The most common file systems can be made with programs in /sbin that start with "mk" like mkfs.msdos and mke2fs.
To set up a swap partition:
# mkswap -f /dev/hda5 |
# swapon /dev/hda5 |
Mounting a partition means attaching it to the linux file system. To mount a linux partition:
# mount -t ext2 /dev/sda1 /opt |
File system type. Other types you are likely to use are:
msdos (DOS)
hfs (mac)
iso9660 (CDROM)
nfs (network file system)
Device name. Other device names you are likely to use:
/dev/hdb2 (second partition in second IDE drive)
/dev/fd0 (floppy drive A)
/dev/cdrom (CDROM)
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.
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 |
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.