Hardware Reference
In-Depth Information
Now let's say that drive #2 of the set fails. Looking at the other data you have,
you know that the first number is 1, the third number is 1, and the parity is
even. With that information, you can deduce that the missing number is 0. Voilà,
you've just reconstructed that bit. When RAID stripes data across multiple drives,
it alternates which drive will hold the parity bit so that the data is evenly spread
out across all drives.Then, if one of the drives ever needs to be replaced, a utility
can reconstruct every bit that the old drive contained.That's the general concept,
but in all except RAID 2, the data is striped in 64KB chunks, rather than individual
bits. Working with larger blocks of data makes the operations go faster.
Low-Level Formatting
When a disk is manufactured, its surface is one large area. Before the disk can accept any
data, an organizational structure must be imposed that uniquely names each physical
location on the disk. That way, the drive controller can specify the exact physical spot
where a given bit of data should be written or retrieved.
Here are the organizational units into which a disk is divided:
Heads
Almost all magnetic disks are double-sided. There is a separate read/write head for
each side. On hard drives, there are usually multiple stacked platters, each with a top and
bottom head.
Tracks
Each disk side is organized into concentric rings called
tracks
, like the rings on a
cross-section of a tree. A high-capacity hard disk can have tens of thousands of tracks.
Cylinders
The read/write heads move in and out on a single actuator arm so that all the
heads are in the same in-out position at all times. The stacks of tracks accessible at a given
arm position is a cylinder. The number of cylinders a drive contains is the same as the
number of tracks on a single disk side.
Sectors
The surface of the disk is further divided into pie slices made by lines that cross
over the track lines. Where these lines intersect the track lines, they create small segments
called
sectors
. Sectors are uniquely numbered, and they can be referred to by their numbers
when their data is needed. Each sector holds 512 bytes on most disks.
This slicing up of the disk surface into logical organizational units is accomplished by a
procedure known as low-level formatting.
Low-level formatting
determines the number and
spacing of the tracks and the number and spacing of sectors per track. Together, these two
factors determine the total number of sectors on the disk and therefore its storage capacity.
Hard disks are low-level formatted at the factory. A label on a hard drive casing reports
the number of cylinders, heads, and sectors for the disk, sometimes abbreviated
CHS
.
Another name for the drive's organizational characteristics is
geometry
. BIOS setup requires
this information in order to communicate with the drive.
In the early days of hard disks, the CHS values reflected the physical reality
of how a disk was low-level formatted. Today, however, that is no longer
the case. A drive's CHS values don't necessarily refer to the exact physical
arrangement on the drive, but to the logical way that the drive controller
sees the drive as it communicates with it. For example, a drive's label might
report that it has 16 heads (8 platters) when it actually has only 3 or 4.
