Hardware Reference
In-Depth Information
At the software level, disk cache programs such as SMARTDRV (DOS) and VCACHE (Windows)
can have a major effect on disk drive performance. These cache programs hook into the BIOS hard
drive interrupt and intercept the read and write calls to the disk BIOS from application programs and
device drivers.
When an application program wants to read data from a hard drive, the cache program intercepts the
read request, passes the read request to the hard drive controller in the usual way, saves the data read
from the disk in its cache memory buffer, and then passes the data back to the application program.
Depending on the size of the cache buffer, data from numerous sectors can be read into and saved in
the buffer.
When the application wants to read more data, the cache program again intercepts the request and
examines its buffers to see whether the requested data is still in the cache. If so, the program passes
the data back from the cache to the application immediately, without another hard drive operation.
Because the cached data is stored in memory, this method speeds access tremendously and can greatly
affect disk drive performance measurements.
Virtually all drives have varying amounts of cache memory built directly into the drive's onboard
controller. I remember the days when 1MB or 2MB of RAM was a lot of memory for an entire
system. Today, low-end drives might feature 2-8MB, with mid-range and high-performance drives
typically include 16-64MB of cache.
Although software and hardware caches can make a drive faster for routine or repetitive data transfer
operations, a cache will not affect the true maximum transfer rate the drive can sustain.
Interleave Selection
In a discussion of disk performance, the issue of interleave was once a common concern. Although
traditionally this was more a controller performance issue than a drive issue, modern ATA HDDs
with built-in controllers are fully capable of processing the data as fast as the drive can send it. In
other words, all modern ATA drives are formatted with no interleave (sometimes expressed as a 1:1
interleave ratio). On older hard drive types, such as MFM and ESDI, you could modify the interleave
during a low-level format to optimize the drive's performance. Today, drives are low-level formatted
at the factory, and interleave adjustments are a moot topic.
Reliability
When you shop for a drive, you might notice a statistic called the mean time between failures (MTBF)
described in the drive specifications. MTBF figures usually range from 300,000 to 1,000,000 hours
or more. I usually ignore these figures because they are derived theoretically.
In understanding the MTBF claims, you must understand how the manufacturers arrive at them and
what they mean. Most manufacturers have a long history of building drives, and their drives have seen
millions of hours of cumulative use. They can look at the failure rate for previous drive models with
the same components and calculate a failure rate for a new drive based on the components used to
build the drive assembly. For the electronic circuit board, they also can use industry-standard
techniques for predicting the failure of the integrated electronics. This enables them to calculate the
predicted failure rate for the entire drive unit.
To understand what these numbers mean, you must know that the MTBF claims apply to a population
of drives, not an individual drive. This means that if a drive claims to have an MTBF of 500,000
hours, you can expect a failure in that population of drives in 500,000 hours of total running time. If
