Hard disk limitations
Up through the years, hard disks have suffered from a long list of software related limitations. Read about
that under the file system.
Hard disk speeds
There are countless test programs and measuring methods to evaluate the various hard disks.
Don't place too much stock in the sales person's presentation of seek times. Many hard disks are
advertised with a number like 8 ms. That refers to a seek time, which is measured in milliseconds. There
are many different seek times. That makes comparison difficult. You can measure in terms of:
Average track to track speed. How long does it take the actuator to move read/write heads from one
track to another? There are typically 3000 tracks on a platter side. There, a track change could be to
just one over. That might take 2 ms. Or, it could be up to 2999 tracks over. That might take 20 ms.
On current hard disks, the average seek time will be between 8 and 14 ms.
Change time between read and write - That takes time also.
Wait time for the right sector. When the arm moves to a track, it must wait for the right sector to
appear under the head. That takes time also. On the average, the platter must rotate ½ revolution, to
reach the right sector. This time is directly proportional to the disk rotation speed. On modern hard
disks it usually is between four and eight ms.
When I test a hard disk, I emphasize practical applications. You can take a stopwatch and measure, for
example how long it takes to start Windows 95 or Word 97 (possibly including a large file). That type of
measurement can really tell you something about the hard disk's performance. However they must be
under under comparable circumstances. System board, CPU, and the driver program also influence the
results.
Up through the years, hard disks have suffered from a long list of software related limitations. Read about
that under the file system.
Hard disk speeds
There are countless test programs and measuring methods to evaluate the various hard disks.
Don't place too much stock in the sales person's presentation of seek times. Many hard disks are
advertised with a number like 8 ms. That refers to a seek time, which is measured in milliseconds. There
are many different seek times. That makes comparison difficult. You can measure in terms of:
Average track to track speed. How long does it take the actuator to move read/write heads from one
track to another? There are typically 3000 tracks on a platter side. There, a track change could be to
just one over. That might take 2 ms. Or, it could be up to 2999 tracks over. That might take 20 ms.
On current hard disks, the average seek time will be between 8 and 14 ms.
Change time between read and write - That takes time also.
Wait time for the right sector. When the arm moves to a track, it must wait for the right sector to
appear under the head. That takes time also. On the average, the platter must rotate ½ revolution, to
reach the right sector. This time is directly proportional to the disk rotation speed. On modern hard
disks it usually is between four and eight ms.
When I test a hard disk, I emphasize practical applications. You can take a stopwatch and measure, for
example how long it takes to start Windows 95 or Word 97 (possibly including a large file). That type of
measurement can really tell you something about the hard disk's performance. However they must be
under under comparable circumstances. System board, CPU, and the driver program also influence the
results.
No comments:
Post a Comment