[16 November 2004}: Capturing signals using PC based instrumentation at tens or hundreds of
megabytes per second inevitably hits a bottleneck when continuous streaming to
the hard disk is required.
It is for this reason that large memory is often installed onto acquisition boards,
but where gigabytes of data are involved even this is not sufficient. So is
there a way forward? Until recently IDE hard drives for the standard PC and notebook
plugged into PATA (Parallel Advanced Technology Attachment) ribbon cable.
This interface has been steadily improved in terms of clock speed but hits a
problem due to interference between conductors making up the wide data ribbon
cables, which is worst at high clock speeds.
To combat this development of drives that can use SATA (Serial Advanced
Technology Attachment) is seen as a way forward, where serial transmissions run
across a single control channel compared to the multiple channels of a parallel
interface. This means that at the same clock speeds, the serial line will carry
less data, but because the serial method requires fewer wires, less interference
is generated to cause data integrity problems. This allows for serial
transmission methods to run at much higher speeds than the equivalent parallel
methods. Companies such as Dell are now starting to fit these drives as standard
into many of their computers, and a suitable controller card can be retrofitted
into older PCs. So is this a major jump in performance?
Evidence suggests a slight improvement but there are other factors such as a
hard drives cache size, spindle speed and access times to take into account.
Having a lightening fast connection bus connection is not the "be and end all"
however RAID systems do show much promise.RAID stands for redundant arrays of
inexpensive disks, but the word "redundant" might be a little misleading.
In fact RAID usefully combines multiple small, inexpensive disk drives into an
array of disk drives that yields performance and data security benefits which
can exceed that of a single large (more expensive) drive.
This array of drives appears to the computer as a single logical storage unit
or drive, but it must be noted that there is no gain in storage size in this
arrangement, for example using two 80Gbyte drives will yield 80Gbyte.
The key to increased performance under RAID is parallelism, where simultaneous
access to multiple disks allows data to be written to or read from a RAID array
faster than would be possible with a single drive.
RAID is commonly available in configurations RAID 0, 1, 2, 3, 4, 5 or 10 (with
more are being added over time), but how to choose?
Here we will look closer at systems 0 and 1, both of which will work with just
two drives, and represent the entry level system most applicable for a PC based
instrumentation system. RAID Level 0. At this level, data is split across drives by a process called
"striping", resulting in higher data throughput. As no redundant information is
stored, performance is very good and can be expected to at least double that of
a single drive, but the failure of any disk in the array results in data loss.
RAID Level 1 provides redundancy by writing all data to two (or more) drives in
a "mirroring" process. As the data are identical on each drive, having a
redundant drive has the advantage of always having a copy of the data safe.
The performance of a level 1 array tends to be faster on reads and slower on
writes compared to a single drive or Raid 0, but if either drive fails, no data
are lost. The choice really comes down to which is the most important to your
application performance or data security. Most users of PC instrumentation will
go for RAID 0 as the best and most viable way to capture gigabytes of data at
the highest speed, with quality hard drives minimising the possibility of data
loss through drive failure. There is a way of combining RAID 1 and 0 to get the
best of both worlds, but four drives are required. The common way to undertake
connection of two drives into a RAID system is by use of a controller card.
If there is room internally for two drives in the PC a PCI controller card may
be used, with onboard connections for internal wiring. Where space is at a
premium an external box to hold the drives is a possibility, the connection
being via SCSI or fibre channel, the latter being only of real advantage if you
have more than five drives, though it can communicate to 10km if converted to
optical cable. However the controller card should be considered as the simplest
and most effective method for just two drives and can be configured for RAID 0 or 1.
Such cards usually support the main operating systems Windows XP, 2000, Red Hat
and SuSE Linux. The entry level system consisting of one such controller card
plus two 120Gbyte SATA drives currently costs in the region of GBP 150 to GBP
200 plus VAT, it really depends on the storage capacity and the quality of the
drives. At these prices it's a good investment not only for ultra high speed PC
instrumentation, but for everyday office use too. If it is a ultra high speed
signal capture system you are after, Dataquest Solutions can provide further
advice on choosing the best system to meet your requirements.
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