What is RAID?

What is RAID (Redundant Array of Independent Disks)?
RAID creates a single usable data disk, where several physical disks are combined into an array for better speed and/or fault tolerance. There are three key concepts in RAID: mirroring, the copying of data to more than one disk; striping, the splitting of data across more than one disk; and error correction, where redundant data is stored to allow problems to be detected and possibly fixed (known as fault tolerance). Although there are many different levels of RAID.

RAID 0 (Striped set without parity/Non-Redundant Array) Implements data striping where file blocks are written across multiple drives in fragments and requires a minimum of 2 disks. The advantage of a RAID 0 is that the read/write speed is dramatically increased. The more disks in the array the greater the bandwidth. The disadvantage to a RAID0 is that there is no fault tolerance; if a single drive fails it will destroy the array. Also a RAID 0 does not implement error checking so any error is also unrecoverable. A common solution to this is to have a drive outside of the array used as back-up storage in case of a hardware failure.

RAID 1 (Mirrored set without parity) Implements data mirroring. Data is duplicated on two or four drives through a hardware raid controller and provides some fault tolerance. The array is recoverable as long as at least 1 drive has not failed. It provides faster read performance than a single drive and provides drive redundancy in case of drive failure. There is also a very slight reduction to write speed.

RAID 5 (Striped set with dual distributed parity) Implements data striping at a block level, and distributes parity among the drives. The parity information allows recovery from the failure of any single drive because any following reads can be calculated from the distributed parity. Another advantage of a Raid 5 allows for increased read/write speeds while allowing the most efficient use of disk space. RAID 5 requires a minimum of 3 disks.

RAID 10 (RAID 1 + 0) Creates multiple mirrors, where data is organized as stripes across multiple disks and then the striped disk sets are mirrored. RAID 10 offers the same fault tolerance as RAID 1 with increased read/write speeds over a single Raid 1 volume or single drive. RAID Level 10 requires 4 drives to implement.


How do I check the health of my 3ware RAID array?

3ware allows for a browser interface to be used, however unless accessed locally this can be a security risk. Therefore we suggest using the command line interface if at all possible.

You need to download the 3ware CLI utilities from the vendor website or from your datacenter portal. Installation is as simple as unzipping the file into a separate folder. Please note you MUST be root/administrator to run the utility.

Quick command reference for 3ware CLI tools

These devices must be followed by a number denoting which is being queried.

tw_cli /c0 show (Output shows information needed to know the health of the RAID array)

./tw_cli /c1 show


Unit  UnitType  Status         %Cmpl  Stripe  Size(GB)  Cache  AVerify  IgnECC


u0    RAID-5    OK             –      64K     465.641   OFF    OFF      OFF

Port   Status           Unit   Size        Blocks        Serial


p0     OK               u0     233.76 GB   490234752     WD-WCANY1727093

p1     OK               u0     233.76 GB   490234752     WD-WCANY1622544

p2     OK               u0     233.76 GB   490234752     WD-WCANY1657267

p3     NOT-PRESENT      –      –           –             –

*note the following:

c = controller
Controller can be 0 or 1
u = unit
Unit number depends on number arrays. It is 0 in most cases.
p = port
Port denotes port number. In most cases, it is 0-4.