CS170 Practice Question on File systems

Disk performance

The disk rotates at 12,000 RPM (rotations per minute)

The disk has 10 platters (and 10 corresponding heads); the cost to change which head is active is zero

Each sector is 512 bytes

There are 1024 sectors per track (we are ignoring the fact that the number of sectors per track varies on a real disk)

There are 4096 tracks per platter

The track-to-track seek time is 0 milliseconds

If the disk head has to seek more than a single track, the seek time is given by 1 + t(.003) milliseconds, where t is the number of tracks that the disk is seeking over.

Ignore the time to transfer the bits from the disk to memory; that is, once the disk head is positioned over the sector, the transfer happens instantaneously.

What is the storage capacity of the disk in bytes or gigabytes? (Explain briefly.)
What is the sequential transfer bandwidth, expressed in bytes/second or megabytes/second? (Explain briefly.)
Now assume that the disk with the above characteristics is given a never-ending stream of requests to read one sector at a time, with each request chosen randomly from all possible sectors on the disk. Assume that these read requests are scheduled in FIFO order. State the effective long-term transfer rate that the disk can sustain, expressed in bytes/second or kilobytes/second, and explain briefly.

In doing the third question, the following may be useful:

You can (and probably should) make several percentage point approximations, for example 4096 can be represented as 4000, and 13 x 7.5 is approximately 100.
If we pick a track uniformly at random and then pick another track uniformly at random, the expected (or average) number of tracks between them is one-third (not one-half) the total number of tracks on a platter (this can be derived using probability theory; OSTEP also gives a derivation.
The term "long-term transfer rate" refers to R/X, where R is the number of bytes to transfer in each read, and X is the average length of time that a read takes.

File systems

The disk is divided into 1024-byte blocks.

The beginning of the disk contains an array of 2^{16} inodes, each of which can represent a file or be unallocated.

A file has an indexed structure: an inode contains (a) 8 data block pointers, each of which is 4 bytes and each of which points to a disk block and (b) a pointer to ONE indirect block, which is a disk block that itself contains data block pointers.

The inode also contains a userid (2 bytes), three time stamps (4 bytes each), protection bits (2 bytes), a reference count (3 bytes), and the size (4 bytes).

A directory contains a list of

(file_name,
inode_number)

pairs, where the file_name portion is always exactly 14 bytes, including the null terminator (if the file_name would otherwise be fewer than 14 bytes, it is padded to 14 bytes).

State the maximum file size, and explain briefly, for example by showing your work. You may express your answer as a sum of powers-of-two.
State the maximum number of files in a directory, and explain briefly, for example by showing your work. Again, you may express your answer as a sum of powers-of-two.

CS170 Practice Questions on Disks, File systems

CS170 Practice Questions on File systems

Disk performance

File systems