Hardware Reference
In-Depth Information
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 recent ATA, SCSI, and SASI 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 ATA drives might fea-
ture 2-8MB, with mid-range and high-performance ATA, SCSI, SASI, and FibreChannel
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
asMFMandESDI,youcouldmodifytheinterleaveduringalow-levelformattooptimize
the drive's performance. Today, drives are low-level formatted at the factory, and inter-
leave 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 de-
rived theoretically.
In understanding the MTBF claims, you must understand how the manufacturers arrive at
them andwhat theymean. Mostmanufacturers havealonghistory ofbuilding 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
 
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