7.2.1.1. 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.

7.2.1.2. Java File Functions¶

RandomAccessFile(String name, String mode)

close()

read(byte[] b)

write(byte[] b)

seek(long pos)

7.2.1.3. Primary vs. Secondary Storage¶

• Primary storage: Main memory (RAM)

• Secondary Storage: Peripheral devices

  • Disk drives

  • Tape drives

  • Flash drives

7.2.1.4. 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.

7.2.1.5. 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.

7.2.1.6. Disk Drives¶

7.2.1.7. Sectors¶

• A sector is the basic unit of I/O.

7.2.1.8. 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.

7.2.1.9. 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.

7.2.1.10. 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.

7.2.1.11. (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

7.2.1.12. 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

7.2.1.13. 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.

7.2.1.14. 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

7.2.1.15. 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

7.2.1.16. 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.