Q: How do I know if I have a RAID card on my 2008 Power Mac running OS X Server?

If you are running OS X Server, then it's unlikely you have a PowerMac. PowerMacs were no longer in production after 2005.

OS X has built-in RAID software that is part of Disk Utility. Mac Pros did not come with a hardware RAID card.

How to rebuild a software RAID mirror

RAID Basics

 

For basic definitions and discussion of what a RAID is and the different types of RAIDs see RAIDs.  Additional discussions plus advantages and disadvantages of RAIDs and different RAID arrays see:

 

RAID Tutorial;

RAID Array and Server:

Hardware and Service Comparison.

Hardware or Software RAID?

 

RAID Hardware Vs RAID Software - What is your best option?

 

RAID is a method of combining multiple disk drives into a single entity in order to improve the overall performance and reliability of your system. The different options for combining the disks are referred to as RAID levels. There are several different levels of RAID available depending on the needs of your system. One of the options available to you is whether you should use a Hardware RAID solution or a Software RAID solution.

 

RAID Hardware is always a disk controller to which you can cable up the disk drives. RAID Software is a set of kernel modules coupled together with management utilities that implement RAID in Software and require no additional hardware.

 

Pros and cons

 

Software RAID is more flexible than Hardware RAID. Software RAID is also considerably less expensive. On the other hand, a Software RAID system requires more CPU cycles and power to run well than a comparable Hardware RAID System. Also, because Software RAID operates on a partition by partition basis where a number of individual disk partitions are grouped together as opposed to Hardware RAID systems which generally group together entire disk drives, Software RAIDs are slightly more complicated to run. This is because they have more available configurations and options. An added benefit to the slightly more expensive Hardware RAID solution is that many Hardware RAID systems incorporate features that are specialized for optimizing the performance of your system.

 

For more detailed information on the differences between Software RAID and Hardware RAID you may want to read: Hardware RAID vs. Software RAID: Which Implementation is Best for my Application?

Do You Really Need a RAID?

 

There is only one thing a RAID  provides - more space.  Beyond that a RAID can’t help you with:

 

1. Accidental deletion or user error
2. Viruses or malware
3. Theft or catastrophic damage
4. Data corruption due to other failed hardware or power loss
5. Striped RAIDs have a higher failure risk than a single drive

 

The purpose of a RAID is to provide high speed mass storage for specialized needs like video editing, working with extremely large files, and storing huge amounts of data.

 

If your array fails it means complete loss of data and hours of time to rebuild.  RAIDs degrade over time necessitating many hours of restoration.  And, if you don't know much about RAIDs then you really don't need one.

 

You can use a RAID for backup.  But unless your backup needs involve TBs of data requiring rapid and frequent access, why bother?  TM works in the background.  It's not like you have to sit there waiting for your backup to be completed.  Furthermore, you're buying two drives possibly to solve a problem where a single drive will do.  And, one drive is less expensive than two.

 

Ignoring overhead, two drives in a RAID 0 (striped) array should perform about twice as fast. However, as the array fills up with files that performance will degrade.

 

RAID was a technology that in it's time was meant to solve a problem.  Large capacity, fast drives were extremely expensive.  Small drives were cheaper but slower.  However, combining these cheaper drives into arrays gave faster performance and the larger capacity needed for data storage needs.  Thus, the reason why it's called Redundant Array of Inexpensive Drives.  But today you can buy a 3 TB drive with performance that's better than the 1 TB drives of two or three years ago.

 

But why trust your boot drive to a RAID?  I certainly wouldn't.

RAID

(Redundant Array of Independent Disks) A disk subsystem that is used to increase performance or provide fault tolerance or both. RAID uses two or more ordinary hard disks and a RAID disk controller. In the past, RAID has also been implemented via software only.

In the late 1980s, the term stood for "redundant array of inexpensive disks," being compared to large, expensive disks at the time. As hard disks became cheaper, the RAID Advisory Board changed "inexpensive" to "independent."

 

Small and Large

RAID subsystems come in all sizes from desktop units to floor-standing models (see NAS and SAN). Stand-alone units may include large amounts of cache as well as redundant power supplies. Initially used with servers, desktop PCs are increasingly being retrofitted by adding a RAID controller and extra IDE or SCSI disks. Newer motherboards often have RAID controllers.

Disk Striping

RAID improves performance by disk striping, which interleaves bytes or groups of bytes across multiple drives, so more than one disk is reading and writing simultaneously.

Mirroring and Parity

Fault tolerance is achieved by mirroring or parity. Mirroring is 100% duplication of the data on two drives (RAID 1). Parity is used to calculate the data in two drives and store the results on a third (RAID 3 or 5). After a failed drive is replaced, the RAID controller automatically rebuilds the lost data from the other two. RAID systems may have a spare drive (hot spare) ready and waiting to be the replacement for a drive that fails.

The parity calculation is performed in the following manner: a bit from drive 1 is XOR'd with a bit from drive 2, and the result bit is stored on drive 3 (see OR for an explanation of XOR).

RAID Levels

RAID 0 - Speed
Level 0 is disk striping only, which interleaves data across multiple disks for better performance. It does not provide safeguards against failure. RAID 0 is widely used in gaming machines for higher speed.

 

RAID 1 - Fault Tolerance
Uses disk mirroring, which provides 100% duplication of data. Offers highest reliability, but doubles storage cost. RAID 1 is widely used in business applications.

RAID 2 - Speed
Bits (rather than bytes or groups of bytes) are interleaved across multiple disks. The Connection Machine used this technique, but this is a rare method.

RAID 3 - Speed and Fault Tolerance
Data are striped across three or more drives. Used to achieve the highest data transfer, because all drives operate in parallel. Parity bits are stored on separate, dedicated drives.

RAID 4 - Speed and Fault Tolerance
Similar to Level 3, but manages disks independently rather than in unison. Not often used.

RAID 5 - Speed and Fault Tolerance
Data are striped across three or more drives for performance, and parity bits are used for fault tolerance. The parity bits from two drives are stored on a third drive and are interspersed with user data. RAID 5 is widely used on servers to provide speed and fault tolerance.

 

RAID 6 - Speed and Fault Tolerance
Highest reliability, but not widely used. Similar to RAID 5, but performs two different parity computations or the same computation on overlapping subsets of the data.

 

RAID 10 - Speed and Fault Tolerance
A combination of RAID 1 and RAID 0 combined. Raid 0 is used for performance, and RAID 1 is used for fault tolerance.

[Permission to use part or all of the above has been granted by Kappy, exclusively, to theratter.]