In the last couple of years, hard disk manufacturers have dramatically increased the
size of the hard disk buffers in their products. Even as recently as the late 1990s, 256
to 512 kiB was common on consumer
drives, and it was not unusual to find only 512 kiB buffers on even some SCSI units
(though many had from 1 MiB to 4 MiB). Today, 2 MiB buffers are common on retail IDE/ATA
drives, and some SCSI drives are now available with a whopping 16 MiB!
I believe there are two main reasons for this dramatic increase in buffer sizes. The
first is that memory prices have dropped precipitously over the last few years. With the
cost of memory only about $1 per MiB today, it doesn't cost much to increase the amount
the manufacturers put into their drives. The second is related to marketing: hard disk
purchasers have a perception that doubling or quadrupling the size of the buffer will have
a great impact on the performance of the hardware.
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The cache chip from a Seagate Barracuda hard disk
logic board. This chip is the entire cache: it's a 4 Mib chip,
which is 512 kiB, the size of the cache on this drive. Some
caches use more than one chip, especially the larger ones. |
The size of the disk's cache is important to its overall impact in improving
the performance of the system, for the same reason that adding system memory will improve system performance, and why
increasing the system cache will improve
performance as well. However, the attention that the size of the hard disk buffer is
getting today is largely unwarranted. It has become yet another "magic number"
of the hardware world that is tossed around too loosely and overemphasized by salespeople.
In fact, a
benchmarking comparison done by StorageReview.com showed very little performance
difference between 512 kiB and 1 MiB buffer versions of the same Maxtor hard drive. See this section for more on this performance
metric.
So, where does this leave us? Basically, with the realization that the size of the
buffer is important only to an extent, and that only large differences (4 MiB vs. 512 kiB)
are likely to have a significant impact on performance. Also remember that the size of the
drive's internal buffer will be small on most systems compared to the amount of system
memory set aside by the operating system for its disk
cache. These two caches, the one inside the drive and the one the operating system
uses to avoid having to deal with the drive at all, perform a similar function, and really
work together to improve performance.
Next: Write Caching
