How A Hard Drive Works
A hard disk uses rigid rotating platters (disks). Each
platter has a planar magnetic surface on which digital data may be stored.
Information is written to the disk by transmitting an electromagnetic
flux through an antenna or read-write head that is very close to a magnetic
material, which in turn changes its polarization due to the flux. The
information can be read by a read-write head
which senses electrical change as the magnetic fields pass by in close
proximity as the platter rotates.
A typical hard disk drive design consists of a central axis or spindle
upon which the platters spin at a constant rotational velocity. Moving
along and between the platters on a common armature are read-write heads,
with one head for each platter face. The armature moves the heads radially
across the platters as they spin, allowing each head access to the entirety
of the platter.
The associated electronics control the movement of the read-write armature
and the rotation of the disk, and perform reads and writes on demand from
the disk controller. Modern drive firmware is capable of scheduling reads
and writes efficiently on the disk surfaces and remapping sectors of the
disk which have failed.
Also, most major hard drive and motherboard vendors now support S.M.A.R.T.
technology, by which impending failures can often be predicted, allowing
the user to be alerted in time to prevent data loss.
The (mostly) sealed enclosure protects the drive internals from dust,
condensation, and other sources of contamination. The hard disk's read-write
heads fly on an air bearing (a cushion of air) only nanometres above the
disk surface. The disk surface and the drive's internal environment must
therefore be kept immaculately clean to prevent damage from fingerprints,
hair, dust, smoke particles, etc. given the submicroscopic gap between
the heads and disk.
Contrary to popular belief, a hard disk drive does not contain a vacuum.
Instead, the system relies on air pressure inside the drive to support
the heads at their proper flying height while the disk is in motion. Another
common misconception is that a hard drive is totally sealed. A hard disk
drive requires a certain range of air pressures in order to operate properly.
If the air pressure is too low, the air will not exert enough force on
the flying head, the head will not be at the proper height, and there
is a risk of head crashes and data loss. (Specially manufactured sealed
and pressurized drives are needed for reliable high-altitude operation,
above about 10,000 feet. This does not apply to pressurized enclosures,
like an airplane cabin.) Modern drives include temperature sensors and
adjust their operation to the operating environment.
Hard disk drives are not airtight. They have a permeable filter (a breather
filter) between the top cover and inside of the drive, to allow the pressure
inside and outside the drive to equalize while keeping out dust and dirt.
The filter also allows moisture in the air to enter the drive. Very high
humidity year-round will cause accelerated wear of the drive's heads (by
increasing stiction, or the tendency for the heads to stick to the disk
surface, which causes physical damage to the disk and spindle motor).
You can see these breather holes on all drives -- they usually have a
warning sticker next to them, informing the user not to cover the holes.
The air inside the operating drive is constantly moving too, being swept
in motion by friction with the spinning disk platters. This air passes
through an internal filter to remove any leftover contaminants from manufacture,
any particles that may have somehow entered the drive, and any particles
generated by head crash.
Due to the extremely close spacing of the heads and disk surface, any
contamination of the read-write heads or disk platters can lead to a head
crash — a failure of the disk in which the head scrapes across the
platter surface, often grinding away the thin magnetic film. For Giant
Magnetoresistive (GMR) heads in particular, a minor head crash from contamination
(that does not remove the magnetic surface of the disk) will still result
in the head temporarily overheating, due to friction with the disk surface,
and renders the disk unreadable until the head temperature stabilizes.
Head crashes can be caused by electronic failure, a sudden power failure,
physical shock, wear and tear, or poorly manufactured disks. Normally,
when powering down, a hard disk moves its heads to a safe area of the
disk, where no data is ever kept (the landing zone). However, especially
in old models, sudden power interruptions or a power supply failure can
result in the drive shutting down with the heads in the data zone, which
increases the risk of data loss. Newer drives are designed such that the
rotational inertia in the platters is used to safely park the heads in
the case of unexpected power loss. IBM pioneered drives with "head unloading"
technology that lifts the heads off the platters onto "ramps" instead
of having them rest on the platters, reducing the risk of stiction. Other
manufacturers also use this technology.
Apple has created a technology for their new PowerBook line of laptop
computers called Sudden Motion Sensor, or SMS. When an instant movement
detected by the built-in motion sensor in the PowerBook, internal hard
disk heads automatically unload themselves into the parking zone to reduce
the risk of any potential data loss or scratches made.
Spring tension from the head mounting constantly pushes the heads towards
the disk. While the disk is spinning, the heads are supported by an air
bearing and experience no physical contact wear. The sliders (the part
of the heads that are closest to the disk and contain the pickup coil
itself) are designed to reliably survive a number of landings and takeoffs
from the disk surface, though wear and tear on these microscopic components
eventually takes its toll. Most manufacturers design the sliders to survive
50,000 contact cycles before the chance of damage on startup rises above
50%. However, the decay rate is not linear — when a drive is younger
and has fewer start/stop cycles, it has a better chance of surviving the
next startup than an older, higher-mileage drive (as the head literally
drags along the drive's surface until the air bearing is established).
For example, the Maxtor DiamondMax series of desktop hard drives are rated
to 50,000 start-stop cycles. This means that no failures attributed to
the head-disk interface were seen before at least 50,000 start-stop cycles
during testing.
Using rigid platters and sealing the unit allows much tighter tolerances
than in a floppy disk. Consequently, hard disks can store much more data
than floppy disk, and access and transmit it faster. In 2005, a typical
workstation hard disk might store between 80 GB and 500 GB of data, rotate
at 7,200 to 10,000 rpm, and have a sequential transfer rate of over 50
MB/s. The fastest workstation and server hard drives spin at 15,000 rpm.
Notebook hard drives, which are physically smaller than their desktop
counterparts, tend to be slower and have less capacity. Most spin at only
4,200 rpm or 5,400 rpm, though the newest top models spin at 7,200 rpm.
A hard disk is generally accessed over one of a number of bus types, including
ATA (IDE, EIDE), Serial ATA, SCSI, SAS, FireWire (aka IEEE 1394), USB,
and Fibre Channel.
Back in the days of the ST-506 interface, the data encoding scheme was
also important. The first ST-506 disks used Modified Frequency Modulation
(MFM) encoding (which is still used on the common "1.44 MB" (1.4 MiB)
3.5-inch floppy), and ran at a data rate of 5 megabits per second. Later
on, controllers using 2,7 RLL (or just "RLL") encoding increased this
by half, to 7.5 megabits per second; it also increased drive capacity
by half.
Many ST-506 interface drives were only certified by the manufacturer to
run at the lower MFM data rate, while other models (usually more expensive
versions of the same basic drive) were certified to run at the higher
RLL data rate. In some cases, the drive was overengineered just enough
to allow the MFM-certified model to run at the faster data rate; however,
this was often unreliable and was not recommended. (An RLL-certified drive
could run on a MFM controller, but with 1/3 less data capacity and speed.)
ESDI also supported multiple data rates (ESDI drives always used 2,7 RLL,
but at 10, 15 or 20 megabits per second), but this was usually negotiated
automatically by the drive and controller; most of the time, however,
15 or 20 megabit ESDI drives weren't downward compatible (i.e. a 15 or
20 megabit drive wouldn't run on a 10 megabit controller). ESDI drives
typically also had jumpers to set the number of sectors per track and
(in some cases) sector size.
SCSI originally had just one speed, 5 MHz (for a maximum data rate of
5 megabytes per second), but later this was increased dramatically. The
SCSI bus speed had no bearing on the drive's internal speed because of
buffering between the SCSI bus and the drive's internal data bus; however,
many early drives had very small buffers, and thus had to be reformatted
to a different interleave (just like ST-506 drives) when used on slow
computers, such as early IBM PC compatibles and Apple Macintoshes.
ATA drives have typically had no problems with interleave or data rate,
due to their controller design, but many early models were incompatible
with each other and couldn't run in a master/slave setup (two drives on
the same cable). This was mostly remedied by the mid-1990s, when ATA's
specfication was standardised and the details begun to be cleaned up,
but still causes problems occasionally (especially with CD-ROM and DVD-ROM
drives, and when mixing Ultra DMA and non-UDMA devices).
Serial ATA does away with master/slave setups entirely, placing each drive
on its own channel (with its own set of I/O ports) instead.
FireWire/IEEE 1394 and USB(1.0/2.0) hard disks are external units containing
generally ATA or SCSI drives with ports on the back allowing very simple
and effective expansion and mobility. Most FireWire/IEEE 1394 models are
able to daisy-chain in order to continue adding peripherals without requiring
additional ports on the computer itself.
There are two modes of addressing the data blocks on more recent hard
disks. The older mode is CHS addressing (Cylinder-Head-Sector), used on
old ST-506 and ATA drives and internally by the PC BIOS. The more recent
mode is the LBA (Logical Block Addressing), used by SCSI drives and newer
ATA drives (ATA drives power up in CHS mode for historical reasons).
CHS describes the disk space in terms of its physical dimensions, data-wise;
this is the traditional way of accessing a disk on IBM PC compatible hardware,
and while it works well for floppies (for which it was originally designed)
and small hard disks, it caused problems when disks started to exceed
the design limits of the PC's CHS implementation. The traditional CHS
limit was 1024 cylinders, 16 heads and 63 sectors; on a drive with 512-byte
sectors, this comes to 504 MiB (528 megabytes). The origin of the CHS
limit lies in a combination of the limitations of IBM's BIOS interface
(which allowed 1024 cylinders, 256 heads and 64 sectors; sectors were
counted from 1, reducing that number to 63, giving an addressing limit
of 8064 MiB or 7.8 GiB), and a hardware limitation of the AT's hard disk
controller (which allowed up to 65536 cylinders and 256 sectors, but only
16 heads, putting its addressing limit at 2^28 bits or 128 GiB).
When drives larger than 504 MiB began to appear in the mid-1990s, many
system BIOSes had problems communicating with them, requiring LBA BIOS
upgrades or special driver software to work correctly. Even after the
introduction of LBA, similar limitations reappeared several times over
the following years: at 2.1, 4.2, 8.4, 32, and 128 GiB. The 2.1, 4.2 and
32 GiB limits are hard limits: fitting a drive larger than the limit results
in a PC that refuses to boot, unless the drive includes special jumpers
to make it appear as a smaller capacity. The 8.4 and 128 GiB limits are
soft limits: the PC simply ignores the extra capacity and reports a drive
of the maximum size it is able to communicate with.
SCSI drives, however, have always used LBA addressing, which describes
the disk as a linear, sequentially-numbered set of blocks. SCSI mode page
commands can be used to get the physical specifications of the disk, but
this is not used to read or write data; this is an artifact of the early
days of SCSI, circa 1986, when a disk attached to a SCSI bus could just
as well be an ST-506 or ESDI drive attached through a bridge (and therefore
having a CHS configuration that was subject to change) as it could be
a native SCSI device. Because PCs use CHS addressing internally, the BIOS
code on PC SCSI host adapters does CHS-to-LBA translation, and provides
a set of CHS drive parameters that tries to match the total number of
LBA blocks as closely as possible.
ATA drives can either use their native CHS parameters (only on very early
drives; hard drives made since the early 1990s use zone bit recording,
and thus don't have a set number of sectors per track), use a "translated"
CHS profile (similar to what SCSI host adapters provide), or run in ATA
LBA mode, as specified by ATA-2. To maintain some degree of compatibility
with older computers, LBA mode generally has to be requested explicitly
by the host computer. ATA drives larger than 8 GiB are always accessed
by LBA, due to the 8 GiB limit described above.
Source: Wikipedia
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