7.Programmer’s View of Files§

Logical view of files:
- An a array of bytes.
- A file pointer marks the current position.
Three fundamental operations:
- Read bytes from current position (move file pointer)
- Write bytes to current position (move file pointer)
- Set file pointer to specified byte position.

Java File Functions

RandomAccessFile(String name, String mode)

close()

read(byte[] b)

write(byte[] b)

seek(long pos)

Primary vs. Secondary Storage

Primary storage: Main memory (RAM)
Secondary Storage: Peripheral devices
- Disk drives
- Tape drives
- Flash drives

Comparisons

\[\begin{split}\begin{array}{l|r|r|r|r|r|r|r} \hline \textbf{Medium}& 1996 & 1997 & 2000 & 2004 & 2006 & 2008 & 2011\\ \hline \textbf{RAM}& \$45.00 & 7.00 & 1.500 & 0.3500 & 0.1500 & 0.0339 & 0.0138\\ \textbf{Disk}& 0.25 & 0.10 & 0.010 & 0.0010 & 0.0005 & 0.0001 & 0.0001\\ \textbf{USB drive}& -- & -- & -- & 0.1000 & 0.0900 & 0.0029 & 0.0018\\ \textbf{Floppy}& 0.50 & 0.36 & 0.250 & 0.2500 & -- & -- & --\\ \textbf{Tape}& 0.03 & 0.01 & 0.001 & 0.0003 & -- & -- & --\\ \textbf{Solid State}& -- & -- & -- & -- & -- & -- & 0.0021\\ \hline \end{array}\end{split}\]

(Costs per Megabyte)
RAM is usually volatile.
RAM is about 1/2 million times faster than disk.

Golden Rule of File Processing

Minimize the number of disk accesses!
1. Arrange information so that you get what you want with few disk accesses.
2. Arrange information to minimize future disk accesses.
An organization for data on disk is often called a file structure.
Disk-based space/time tradeoff: Compress information to save processing time by reducing disk accesses.

Sectors

A sector is the basic unit of I/O.

Terms

Locality of Reference: When record is read from disk, next request is likely to come from near the same place on the disk.
Cluster: Smallest unit of file allocation, usually several sectors.
Extent: A group of physically contiguous clusters.
Internal fragmentation: Wasted space within sector if record size does not match sector size; wasted space within cluster if file size is not a multiple of cluster size.

Seek Time

Seek time: Time for I/O head to reach desired track. Largely determined by distance between I/O head and desired track.
Track-to-track time: Minimum time to move from one track to an adjacent track.
Average Access time: Average time to reach a track for random access.

Other Factors

Rotational Delay or Latency: Time for data to rotate under I/O head.
- One half of a rotation on average.
- At 7200 rpm, this is 8.3/2 = 4.2ms.
Transfer time: Time for data to move under the I/O head.
- At 7200 rpm: Number of sectors read/Number of sectors per track * 8.3ms.

(Old) Disk Spec Example

16.8 GB disk on 10 platters = 1.68GB/platter
13,085 tracks/platter
256 sectors/track
512 bytes/sector
Track-to-track seek time: 2.2 ms
Average seek time: 9.5ms
4KB clusters, 32 clusters/track.
5400RPM

Disk Access Cost Example (1)

Read a 1MB file divided into 2048 records of 512 bytes (1 sector) each.
Assume all records are on 8 contiguous tracks.
First track: 9.5 + (11.1)(1.5) = 26.2 ms
Remaining 7 tracks: 2.2 + (11.1)(1.5) = 18.9ms.
Total: 26.2 + 7 * 18.9 = 158.5ms

Disk Access Cost Example (2)

Read a 1MB file divided into 2048 records of 512 bytes (1 sector) each.
Assume all file clusters are randomly spread across the disk.
256 clusters. Cluster read time is 8/256 of a rotation for about 5.9ms for both latency and read time.
256(9.5 + 5.9) is about 3942ms or nearly 4 sec.

How Much to Read?

Read time for one track: $9.5 + (11.1)(1.5) = 26.2$ ms
Read time for one sector: $9.5 + 11.1/2 + (1/256)11.1 = 15.1$ ms
Read time for one byte: $9.5 + 11.1/2 = 15.05$ ms
Nearly all disk drives read/write one sector (or more) at every I/O access
Also referred to as a page or block

Newer Disk Spec Example

Samsung Spinpoint T166
500GB (nominal)
7200 RPM
Track to track: 0.8 ms
Average track access: 8.9 ms
Bytes/sector: 512
6 surfaces/heads

Buffers

The information in a sector is stored in a buffer or cache.
If the next I/O access is to the same buffer, then no need to go to disk.
Disk drives usually have one or more input buffers and one or more output buffers.