So what do these statistics mean? The first section of figure 1-5 shows the layout for the drive giving formatted capacity (almost, why drive manufacturers insist on not using the standard values for megabytes and gigabytes is beyond me), the interface speed, bytes per sector and number of sectors. This section is fairly self explanatory. What we are really interested in is the second section of figure 1-5 where they provide actual performance statistics.
In the second section of figure 1-5 the following statistics are provided:
In general seek times are controlled by how fast the arm on the actuator can move. This is controlled by the power of the actuator mechanism and the weight of the actuator arm and read/write heads. The more powerful the disk actuator and the stiffer/lighter the actuator arm the lower the seek times.
Latency is controlled by rotational speed of the disk. The higher the disk speed the lower the latency.
Some disk companies also provide expanded statistics, for example figure 1-6 shows the specifications provided by Seagate for their Cheetah series of drives.
To continue reading for free, register below or login
To read more you must become a member of SearchOracle.com
[TABLE]
Figure 1-6: More Detailed Specifications from Seagate
Seagate has added the average write seek times as well as giving formatted transfer rates. This gives a more complete picture of the disks' abilities. Notice that Seagate has also included the number of disks and cylinders as well as the number of heads. The number of heads indicates that the platter is two-sided and both sides are being used for data. The four platters this Seagate drive provide 8 tracks per cylinder (2 on each disk, an upper track and lower track).
However the biggest factors in single disk performance will be the disk speed and speed of the actuator arm as measured by the average latency (rotational latency) and seek times. More platters in a drive will do little to improve write or read time for database related IO. The number of platters is more related to capacity since the read/write heads are generally not independent but are mounted on a single actuator with multiple, parallel arms. This means that only one disk surface is read from or written to per operation but multiple operations may take place in a single IO.
This dependence on the rotational speed and actuator arm speed is what limits the IO capability of the drives. Comparisons of multiple drives from the same company shows that while the number of platters/sides used determines the capacity of the drive, the IO rates and seek and latency values remain fairly constant. Figure 1-7 shows this for multiple Seagate drives with the same speed disk but different capacities.
[TABLE]
Figure 1-7: Platters Effect on IO, Latency and Seek times
To get the same effective IO rate per gigabyte of capacity when going from a single-side, single-platter 40 gigabyte drive to a four-side, dual-platter 160 gigabyte drive you would need four or more of the 160 gigabyte drives (100/40=2.5 MB/sec/GB verses 100/160=0.625 MG/sec/GB)! This is the little secret that undermines the entire "bigger is better" myth of disk capacity.
|
|
|
|
| TechTarget provides technology professionals with the information they need to perform their jobs - from developing strategy, to making cost-effective purchase decisions and managing their organizations' technology projects - with its network of . |
|
| |
All Rights Reserved, , TechTarget |
|
|
|
|
Digg This!
StumbleUpon
Del.icio.us
