Raid Tudor

ActiveServers Raid Tutorial

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. More Information
 
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. More Information
 
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. More Information
 
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. More Information
 
Raid 6 : This configuration uses the same basic idea as RAID 5, but creates two separate parity sets. This means it has to have four disks to function, and loses two disks worth of storage space to parity. However, it also means that any two disks can fail, and the array can still be rebuilt. For example, a 10 disk RAID 6 array would still have 8 disks worth of space and be able to handle two complete disk failures. Reading data from a RAID 6 array is not quite as fast as it would be from a RAID 5 (as there are two parity streams to check), but is still faster than a single disk. More Information
 
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. More Information