SCSI -- Small computer system
interface is one of the oldest forms of
storage interfaces traditionally used in
server or workstation class computers. It's
been through many revisions, from SCSI-1 all
the way up to Ultra-320 SCSI, which is the
modern SCSI interface. (There is an
Ultra-640 standard, but that isn't common.)
The 320 and 640 numbers represent MB/s,
megabytes per second. SCSI-1 started out 5
MB/s. SCSI is still used in modern servers,
but the interface is starting to lose market
share to SAS. Most recent versions of SCSI
can handle up to 15 hard drives.
While the cable sharing mechanism is
relatively efficient, there is a maximum
theoretical cap of 320 MB/s, but that limit
is reduced further by SCSI overhead. It's
theoretically possible that 15 modern SCSI
hard drives could have an aggregate
throughput of 1350 MB/s, so they would be
forced to share a 320 MB/s interface. But in
the vast majority of applications, where
there will inevitably be some random I/O in
the hard drives, the mechanical latency of
the hard drives seeking data means it's
unlikely that an Ultra-320 interface will be
fully saturated.
PATA (IDE)-- Parallel advanced
technology attachment (originally called ATA
and sometimes known as IDE or ATAPI) was the
most dominant desktop computer storage
interface from the late 1980s until
recently, when the SATA interface took over.
PATA hard drives are still being utilized
today, especially in external hard drive
boxes, but they're becoming rare. Some
cheaper high-end server storage devices have
also used PATA. Like SCSI, PATA has also
gone through many revisions. The most recent
version of PATA is UDMA/133 which supports a
throughput of 133 MB/s.
Although PATA supports two devices per
connector in a master/slave configuration,
the performance penalty of sharing a PATA
port is severe and not recommended if
performance is important to the user. The
40-pin connector and cabling is also
extremely wide, which is difficult to use in
a high-density environment and tends to
block proper airflow. The size of the
connector also presents problems for smaller
2.5" hard drives, which require a special
shrunken connector.
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SATA And ESATA-- Serial advanced technology
attachment is the official successor to PATA.
So far, there have been two basic versions
of SATA, with SATA-150 and SATA-300. The
numbers 150 and 300 represent the number of
MB/s that the interfaces support. SATA
doesn't have any performance problems due to
cable/port sharing, but that's because it
doesn't permit sharing at all. One SATA port
permits one device to connect to it. The
downside is that it's much more expensive to
buy an eight-port SATA controller than an
Ultra-320 SCSI controller that allows 15
devices to connect to it. The upside is that
each drive gets a theoretical 300 MB/s.
Current SATA hard drives, however, barely
get 80 MB/s, so the bus interface is a bit
of overkill for now.
SATA uses a small seven-pin connector and a
thin cable, which is more conducive to
denser installations and airflow. That's
important, especially inside a storage array
with 15 hard drives, because you'll need one
port and one cable for every drive, whereas
SCSI lets you hook up one or two ports to
the backplane that the drives attach to.
SATA drives are used in smaller servers and
some less expensive storage arrays.
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SAS -- Serial attached SCSI is the
latest storage interface that's gaining
dominance in the server and storage market.
SAS can be seen as a merged SCSI and SATA
interface, since it still uses SCSI commands
yet it is pin-compatible with SATA. That
means you can connect SAS hard drives or
SATA hard drives or CD/DVD ROM or burner
drives. SAS has a signaling rate of 185,
374, 750, and eventually, 1,500 MB/s. But
storage controller technology has
historically been rated by actual data
throughput, which is lower than the
signaling rate. To make these numbers
comparable to the numbers listed above, the
actual data rates are 150, 300, 600, and
eventually, 1,200 MB/s. Note how the two
lower data rates match up with SATA.
SAS connectors are keyed such that SATA
devices can connect to SAS but SAS devices
can't connect to SATA ports. The ports and
cabling look similar, but SAS cables can be
8 meters long, whereas SATA cabling is
limited to 1 meter. The longer cabling
support is due to higher signal voltages,
but the voltage is dropped to SATA levels
whenever a SATA device is connected.
SAS is designed for the high-end server and
storage market, whereas SATA is mainly
intended for personal computers. Unlike SATA,
SAS can be connected to multiple hard drives
through expanders, but the protocol used to
share a SAS port has lower overhead than
SCSI. Coupled with the fact that the ports
are faster to begin with, SAS offers the
best of SCSI and SATA in addition to
superior performance.
FC -- Fibre channel is both a direct
connect storage interface used on hard
drives and a SAN technology. FC offers
speeds of 100, 200, and 400 MB/s. Native FC
interface hard drives are found in very
high-end storage arrays used in SAN and NAS
appliances, although the technology may
ultimately give way to SAS.
Flash -- Flash memory isn't a storage
interface, but it is used for very high-end
storage applications because it doesn't have
the mechanical latency issues of hard
drives. Flash memory can be packaged into
the shape of a hard drive with any of the
above interfaces so that it can be used in a
storage array. The benefit of flash memory
is that it can offer more than 100 times the
read IOPS (input output per second) and 10
times the write IOPS performance of hard
drives, which is extremely valuable to
database applications.
The downside of flash memory is that it's
very expensive per gigabyte (cost
proportional to the performance advantage)
and it has a limited number of writes and
rewrites. Flash memory will begin to fail
anywhere between 10,000 and 1,000,000
writes. To deal with this limitation, flash
devices use a mechanism called wear leveling
to spread out the damage so that the device
will last longer, but even that has its
limits.
RAM -- Random access memory is also
not traditionally seen as a storage medium,
but it can be used as an ultra-fast storage
device. RAM can be adapted to any of storage
interfaces above to emulate traditional
storage devices connected through SCSI or
ATA, but it can also emulate a storage
device through software called RAM drives.
RAM doesn't suffer the same limited number
of write cycles as flash memory, but it is
by far the most expensive form of storage.
For super high-end storage applications, its
high cost may be justifiable.
