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Raid Tutorial "The correct solution for
your needs?" |
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Defining Your Raid Needs :
Not everyone can afford a High-end
NAS sub systems with multiple clusters. If however,
you are a start-up or small to medium business, this
may not be necessary or is cost prohibitive anyway.
However, it doesn't mean you can't have a great solution.
It only means as a small business you have to be a little
more creative. It also means you will have to work at
it, and knowledge is your best weapon. We feel providing
our clients with all the options and knowledge to make
this easy enough to make an intelligent decision. Our
selections are taken from observations and real world
experience, which offers some guidance.
Let's make sure first we understand
the problem !
How do you protect your server against a hard disk failure?
This tutorial is specifically about understanding Raid
and what level is best suited for your needs. We have
chosen to provide just basic information on the 6 most
popular raid types for entry level servers.
Conclusion Based on cost:
Raid 1: If your funding is limited and
you want to enjoy redundancy, at the lowest possible
cost Raid 1 is the best choice. This Raid Subsystem
requires only two disks.
Raid 5: Is the number two choice, as it
is low cost and also a very versatile raid sub system.
Since it only requires a minimum of 3 disks to build
this. It's negative is that is it can be difficult and
time consuming to rebuild.
Raid 10 : It should be stated that these choices
are based on cost and not the preferred Raid subsystem.
Of course Raid 10 is the number one pick really though
it is numbered 3 here. It offers speed ease in rebuilding
and the only negative is drive heat. Also the cost is
a bit higher as it requires 4 disks minimum.
Raid Levels Defined :
Linear Raid : It is
a simple grouping of drives to create a larger virtual
drive. In linear RAID, the chunks are allocated sequentially
from one member drive, going to the next drive only
when the first is completely filled. This grouping provides
no performance benefit, as is unlikely that any I/O
operations will be split between member drives. Linear
RAID also offers no redundancy and, in fact, decreases
reliability — if any one member drive fails, the entire
array cannot be used. The capacity is the total of all
member disks.
Striping : Is the underlying
concept behind all of the other RAID levels. A stripe
is a contiguous sequence of disk blocks. A stripe may
be as short as a single disk block, or may consist of
thousands. The RAID drivers split up their component
disk partitions into stripes; the different RAID levels
differ in how they organize the stripes, and what data
they put in them. The interplay between the size of
the stripes, the typical size of files in the file system,
and their location on the disk is what determines the
overall performance of the RAID subsystem.
Raid 0 : Is much like RAID-linear,
except that the component partitions are divided into
stripes and then interleaved. Like RAID-linear, the
result is a single larger virtual partition. Also like
RAID-linear, it offers no redundancy, and therefore
decreases overall reliability; a single disk failure
will knock out the whole thing. RAID-0 is often claimed
to improve performance over the simpler RAID-linear.
However, this may or may not be true, depending on the
characteristics to the file system, the typical size
of the file as compared to the size of the stripe, and
the type of workload. Making certain of the stripe size
is the correct way to achieve this.
Raid 1 : Is also referred
to as "mirroring". 2 (or more) partitions, all of the
same size, each store an exact copy of all data, disk-block
by disk-block. Mirroring gives strong protection against
disk failure; if one disk fails, there is another with
the an exact copy of the same data. Mirroring can also
help improve performance in I/O-laden systems, as read
requests can be divided up between several disks. Unfortunately,
mirroring is also the least efficient in terms of storage;
two mirrored partitions can store no more data than
a single partition.
Raid 0 + 1 : A hybrid
or combination of RAID levels. For example, if you have
several RAID 1 arrays, where an even number of drives
are striped together, the data is redundant for fault
tolerance, and then stripe those together in a RAID
0. In a RAID 0+1 configuration, the loss of any
drive in a set causes the failure of that entire set
and the set is removed from the RAID system.
Generally (in the two set case) this means you are
left with a RAID 0 system made up of the remaining
set of disks.
Raid 5 : Write performance
is still not as good as for mirroring. Read performance
is also not as good as it is for mirroring, as, after
all, there is only one copy of the data, not two or
more. RAID-5's principle advantage over mirroring is
that it offers redundancy and protection against single-drive
failure, while offering far more storage capacity when
used with three or more drives.
Raid 10 or 1+ 0 :
Is implemented as a striped array whose segments are
RAID 1 arrays. RAID 10 has the same fault tolerance
as RAID level 1. High I/O rates are achieved by striping
RAID 1 segments. Under certain circumstances, RAID 10
array can sustain multiple simultaneous drive failures.
This is an excellent solution for sites that would have
otherwise gone with RAID 1 but need some additional
performance boost.
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