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DADDY BOB'S COMPUTER Q & A
Hard Drive Caching
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By default, disk caching is turned on when
most operating systems are installed.
Caching occurs in both the reads from, and
writes to the hard drive. These two caching
events are similar in some ways, but very
different in others. They are the same in
their overall objective: to untie the fast
PC from the slow mechanics of the hard disk.
The key difference is that a write involves
a
change to the hard disk, while a
read does not.
With no write caching enabled, every write
to the hard disk involves a performance hit
while the system waits for the hard disk to
access the correct location on the hard disk
and write the data. This can takes at least
10-100 milliseconds on the fastest drives,
which is an eternity in the computer world
and really slows down performance as the
system waits for the hard disk. This mode of
operation without a cache used is called
write-through caching. The write
to the disk occurs as soon as the hard drive
is positioned to receive it. This time can
be much longer on some hard drives.
When write caching is enabled, and the
system sends a write to the hard disk, the
logic circuit in the hard drive
stores the data in its much faster cache
memory, and then immediately sends back an
acknowledgement to the operating system
saying, "all done!" The rest of the system
can then proceed on its other business
without having to sit around waiting for the
hard drive to position the read/write arm,
the disk to spin, and so on. This is called
write-back caching, because the
data is stored in the cache and only
"written back" to the drive later on.
Write-back functionality of course improves
performance. There's a catch however. The
drive tells a little white lie when it sends
back "all done" because it really isn’t. The
data isn't on the disk at all, it's only in
the cache. The hard disk's logic circuits
begin to write the data to the disk, but of
course this takes some time. It is sort of
the old “The checks in the mail” routine.
Now, this isn't really a problem most of the
time, as long as
the power stays on. Since cache
memory is volatile, if the power goes out,
its contents are lost. So, any data in the
cache waiting to be written can be lost.
Adding to the problem, the rest of the
system has no way to know this, because it
believed the hard drive when it said it was
done. So not only is some data lost, the
system doesn't even know which data, or even
that it happened. The end result can be
corrupt files, which in the worse case, can
prevent the computer from booting back up
when the power is restored.
At one time, write caching wasn’t used
because of the possible of loosing data.
But it is increasingly being used despite
the risk, as the risk is being greatly
reduced through the use of newer technology.
The most common technique is simply ensuring
that the power does not go off by using a
UPS, Uninterruptible Power Supply.
For added peace of mind, most current drives
have a "write flush" feature that
tells the drive to immediately write to disk
any data held in its cache. This is a
command that is sent just before the system
shuts down for any reason. The power supply
normally takes the 120V household voltage,
and reduces it to 5 and 12V for the hard
drive. When the 120 power is interrupted,
there is sufficient residual power stored in
the power supply's filter capacitors to
continue to supply the 5 and 12 volts to the
hard drive for a long enough time for the
cache to be flushed, and the data written to
the hard drive.
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