RAID
RAID is an acronym for Redundant Array of Inexpensive Disks. Originally, the
idea was that you would get better performance and reliability from several,
less expensive drives linked together as you would from a single, more expensive
drive. The key change in the entire concept is that hard disk prices have
dropped so dramatically that RAID
is no longer concerned with inexpensive
drives. So much so, that the I in RAID
is often interpreted as meaning “Intelligent” or “Independent”, rather than “Inexpensive.”
In the original paper that defined RAID, there were five levels. Since that paper
was written, the concept has been
expanded and revised. In some cases, characteristics of the original levels are
combined to form new levels.
Two concepts are key to understanding RAID.
These are redundancy and parity. The concept of parity is
no different than that used in serial communication, except for the fact that
the parity in a RAID
system can be used to not only detect errors, but correct them. This is because
more than just a single bit is used per byte of data. The parity
information is stored on a drive separate from the data. When an error is detected, the
information is used from the good drives, plus the parity
information to correct the error. It is also possible to have an entire drive fail
completely and still be able to continue working. Usually the drive can be
replaced and the information on it rebuilt even while the system is running.
Redundancy is the idea that all information is duplicated. If you have a system
where one disks is an exact copy of another, one disk is redundant for the
other.
A striped
array is also referred to as RAID
0 or RAID Level 0. Here, portions of the data
are written to and read from multiple disks in parallel. This greatly increases
the speed at which data can be accessed. This is because half of the data is
being read or written by each hard disk, which cuts the access time almost in
half. The amount of data that is written to a single disk is referred to as the
stripe width. For example, if single blocks are written to each disk, then the
stripe width would be a block
This type of virtual disk provides increased
performance since data is being read from multiple disks simultaneously. Since
there is no parity
to update when data is written, this is faster than system
using parity.
However, the drawback is that there is no redundancy. If one disk
goes out, then data is probably lost. Such a system is more suited for
organizations where speed is more important than reliability.
Keep in mind
that data is written to all the physical drives each time data is written to the
logical disk. Therefore, the pieces must all be the same size. For example, you
could not have one piece that was 500 MB and a second piece that was only 400
Mb. (Where would the other 100 be written?) Here again, the total amount of
space available is the sum of all the pieces.
Disk mirroring
(also referred to as RAID
1) is where data from the first drive is duplicated on the second
drive. When data is written to the primary drive, it is automatically written to
the secondary drive as well. Although this slows things down a bit when data is
written, when data is read it can be read from either disk, thus increasing
performance. Mirrored systems are best employed where there is a large database
application
Availability of the data (transaction speed and reliability) is
more important than storage efficiency. Another consideration is the speed of
the system. Since it takes longer than normal to write data, mirrored systems
are better suited to database applications where queries are more common than
updates.
The term used for RAID
4 is a block interleaved undistributed parity
array. Like RAID
0, RAID 4 is also based on striping, but redundancy is built in
with parity
information written to a separate drive. The term “undistributed” is
used since a single drive is used to store the parity
information. If one drive
fails (or even a portion of the drive), the missing data can be created using
the information on the parity
disk. It is possible to continue working even with
one drive inoperable since the parity
drive is used on the fly to recreate the
data. Even data written to the disk is still valid since the parity
information is updated as well. This is not intended as a means of running your system
indefinitely with a drive missing, but rather it gives you the chance to stop
your system gracefully.
RAID 5 takes this one step further and
distributes the parity
information to all drives. For example, the parity drive
for block 1 might be drive 5 but the parity
drive for block 2 is drive 4. With
RAID 4, the single parity
drive was accessed on every single data write, which
decreased overall performance. Since data and parity
and interspersed on a RAID
5 system, no single drive is overburdened. In both cases, the parity
information is generated during the write and should the drive go out, the missing
data can be
recreated. Here again, you can recreated the data while the system is running,
if a hot spare is used. Figure – Raid 5
As I
mentioned before, some of the characteristics can be combined. For example, it
is not uncommon to have stripped arrays mirrored as well. This provides
the speed of a striped array with redundancy of a mirrored array, without the
expense necessary to implement RAID
5. Such a system would probably be referred
to as RAID
10 (RAID 1 plus RAID 0).
Regardless of how long your drives are
supposed to last, they will eventually fail. The question is when. On a server, a
crashed harddisk means that many if not all of your employees are unable to work
until the drive is replaced. However, there are ways of limiting the effects the
crash has in a couple ways. First, you can keep the system from going down
unexpectedly. Second, you can protect the data already on the drive.
The key
issue with RAID
is the mechanisms the system uses to portray the multiple drives
as single one. The two solutions are quite simply hardware and software. With
hardware RAID; the SCSI
host adapter does all of the work. Basically, the
operating system does not even see that there are multiple drives. Therefore,
you can use hardware RAID
with operating systems that do not have any support on their own.
On the other hand software RAID
is less expensive. Linux comes included with
software, so there is no additional cost. However, to me this is no real
advantage as the initial hardware costs are a small fraction of the total cost
of running the system. Maintenance and support play a much larger roll, so these
ought to be considered before the cost of the actual hardware. In it’s Annual
Disaster Impact Research, Microsoft reports that on the average a downed server
costs at least $10,000 per hour. Think about how many RAID
controllers you can buy with that money.
Another advantage of software RAID is the ability to use different types of drives. Although the
partitions need to be the same size, the physical hardware can be different, such as IDE
and SATA.
In addition, the total cost of
ownership also includes user productivity. Should a drive fail, performance
degrades faster with a software solution than with a hardware solution.
Let’s
take an Adaptec AA-133SA RAID
controller as an example. At the time of this writing it is one of the
top end models and provides three Ultra SCSI
channels, which means you could
theoretically connect 45 devices to this single host adapter.
Since each of the channels is Ultra SCSI, you have a
maximum throughput of 120Mbit/s. At the other
end of the spectrum is the Adaptec AAA-131CA, which is designed more for high-end
workstations, as it only supports mirroring
and striping.
One thing to note is that the Adaptec RAID
host adapters do not just provide the interface, which
makes multiple drives appear as one. Instead, they all include a coprocessor,
which increases the performance of the drives considerably.
However, providing data faster and redundancy in not all of it, Adaptec RAID
controllers also have the ability to detect errors and in some cases correct errors on the
hard disk. Many SCSI
systems can already detect single-bit errors. However,
using the parity
information from the drives, the Adaptec RAID
controllers can correct these single-bit errors. In addition,
the Adaptec RAID
controllers can also detect 4-bit errors.
You need to also keep in mind the fact that
maintenance and administration are more costly than the initial hardware. Even
though you have a RAID
5 array, you still need to replace the drive should it
fail. This brings up two important aspects.
First, how well can your system
detect the fact that a drive has failed? Whatever mechanisms you chose must be
in a position to immediately notify the administrators should a drive fail.
The second aspect returns to the fact that maintenance and administration
costs are much higher than the cost of the initial hardware. If the hardware
makes replacing the drive difficult, you increase your downtime and therefore
the maintenance costs increase. Adaptec has addressed this issue by allowing
you to “hot swap” your drives. This means you can replace the defective drive
on a running system, without have to shutdown the operating system.
Note that this also requires that the case containing the RAID
drive be accessible. If your
drives are in the same case as the CPU
(such as traditional tower cases), you
often have difficulty getting to the drives. Removing one while the system is
running is not practical. The solution is an external case, which is
specifically designed for RAID.
Often you can configure the SCSI
ID of the drive with dials on the cases itself and sometimes the position in the case
determines the SCSI
ID. Typically, the drives are mounted onto rails, which
slide into the case. Should one fail, you simple slide it out and replace it
with the new drive.
Protecting your data and being able to replace the drive
is just a start. The next level up is what is referred to as “hot spares.” Here,
you have additional drives already installed that are simply waiting for another
to break down. As soon as a failure is detected, the RAID
card replaces the
failed drive with a spare drive, simply reconfigures the array to reflect the
new drive and the failure is reported to the administrator.
Keep in mind that this must be completely supported in the hardware.
If you have an I/O-bound
application, a failed drive decreases the performance. Instead of just
delivering the data, your RAID
array must calculate the missing data using the
parity information, which means it has a slower response time in delivering the
data. The degraded performance continues until you replace the drive. With a hot
spare, the RAID
array is rebuilding it self as it is delivering data. Although
performance is obviously degraded, it is to a lesser extent than having to swap
the drives manually.
If you have a CPU-bound application,
you obtain
substantial increases in performance over software solutions. If a drive fails,
the operating system
needs to perform the parity
calculations in order to
reconstruct the data. This keeps the CPU
from doing the other tasks and
performance is degraded. Because the Adaptec RAID
controller does all of the
work of reconstructing the data, the CPU
doesn’t even notice it. In fact, even
while the system is running normally, the RAID
controller is doing the appropriate
calculations, so there is no performance lost here either.
In addition, the
Adaptec RAID
controllers can be configured to set the priority of performance
versus availability. If performance is given a high priority, it will take
longer to restore the data. If availability is given the higher priority,
performance suffers. Either is valid, depending on your situation. It is also
possible to give each the same priority.
Because the new drive contains no
data, it must take the time to re-create the data using the parity
information and the data from the other drives. During this time performance will suffer as
the system is working to restore the data on the failed drive.
Redundancy like this can (and therefore the safety of your data) be increased
further by having redundant RAID
5 arrays. For example, you could mirror the entire RAID set. This is often referred to as RAID 51, as it is a
combination of RAID
5 and RAID 1, although RAID 51 was not defined in the
originally RAID
paper. Basically, this is a RAID array which is mirrored. Should a
drive fail, not only can the data be recovered from the parity
information, but it can also be copied from its mirror.
You might also create a RAID 15 array. This is a RAID 5 array, which is made up of mirror sets.
