File attributes; FS implementation ways

File attribute

A file attribute is metadata describing a file: name, type, size, location on disk, owner, protection (permissions) and times of creation, modification and last access.

File system performance indicators

Throughput (data rate), response time/latency, disk access time (seek + rotational delay + transfer), cache hit ratio, disk space utilization, fragmentation, number of disk accesses per operation and reliability.

Ways of implementing a file system (file allocation)

1. Contiguous allocation: a file occupies consecutive blocks; directory holds start and length. Very fast access; suffers external fragmentation and is difficult to extend.
2. Linked-list allocation: each block points to the next. No external fragmentation; poor random access and pointer overhead.
3. Linked list with a table in memory (FAT): pointers kept in a table, which speeds up random access but the table must fit in memory.
4. Indexed allocation (i-node): an index block / i-node lists all the blocks of the file (with indirect blocks for big files). Supports direct access with no external fragmentation.

Which is better and why

Indexed allocation (i-nodes) is generally the best:

• It supports both sequential and direct access efficiently.
• There is no external fragmentation, and files can grow dynamically.
• Only the i-nodes of open files are kept in memory, so memory use is small regardless of disk size (unlike FAT).
• Pointers are collected in one place, improving reliability, and the scheme scales to very large files through indirect blocks.

Contiguous allocation is faster for large read-only files (e.g. CD-ROM), but for general-purpose systems the i-node scheme (used in UNIX, Linux ext2/ext4) gives the best overall balance.