File systems

Vocabulary

English	Meaning
inode	A filesystem record storing file metadata and information locating file data.
journaling	Recording filesystem changes in a log to support recovery after interruption.

Names, inodes, blocks

The inode holds a file's metadata and 4 direct pointers plus one indirect block (256 pointers at 1 KiB each); the directory is itself a file of name-to-inode entries.
Size bound: $(4 + 256) \times 1\,\mathrm{KiB} = 260$ KiB. With 4 KiB blocks: $(4 + 1024) \times 4\,\mathrm{KiB} = 4112$ KiB, about 4 MiB.

{exercise:uni-cs0705-layout}

Practice

With 1 KiB blocks, 4 direct pointers and one 256-pointer indirect block, the max file is —

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Reading block 6 of /notes/x: the directory lookup finds the inode (cached); block 6 is beyond the four direct pointers — read the indirect block (pointer 2), then the data block: two disk reads for one logical block.
Sequentially reading a 100-block file costs $100 + 1$ reads (the indirect block once). Deletion clears pointers, not data — the bytes stay until reused.

{exercise:uni-cs0705-price}

Practice

Reading block 6 (beyond the direct pointers) costs — disk reads

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Append writes data, then the free-list bit, then the inode size. Crash between writes: data-then-inode loses only space (an orphan block); inode-then-data lies about size (reads return garbage).
Journaling writes the intent first and replays it after the crash — the small write tax modern file systems pay for crash consistency.

{exercise:uni-cs0705-crash}

Practice

Journaling survives crashes by —

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