Unit 5 - Computer Architecture
Physically, the hard disk consists of several metallic platters that
are permanently sealed into a disk drive container. The disk drive
includes an access arm assembly “rack” with read / write heads for
each disk surface, as well as a motor which can rotate the disks at
speeds of up to 10,000 rpm.
Binary data on a hard disk is recorded magnetically on invisible, closed concentric circles called “tracks”. To access a particular part of the disk, the read / write heads must first be positioned over the correct track (a seek operation), then they must wait until the required data rotates directly under the read / write heads (a search operation).
Sectors and Clusters
Cluster Size and Performance
When a user wants to save a file, the operating system will allocate disk space in clusters. In other words, when a user wants to save a file, the operating system will use the smallest number of clusters that will hold the file. Therefore, if the cluster size is 2, a 1 MB file would use 1024 clusters; if the cluster size was 64, the same 1 MB file would only use 32 clusters.
The performance of a hard disk is directly related to the cluster size. In general, smaller cluster sizes result in a more efficient use of a hard disk's space, but can also lead to "fragmentation" in large files if the clusters are not stored contiguously (side-by-side) on the hard disk. Larger clusters allow files to grow significantly before another cluster is needed, but will result in large amounts of wasted disk space if files are small.