RAIDS: Back Up Data On Home Servers

Apple Store

Nowadays, when data stored by a common home user is measured by terabytes (thank you, hi-res video and photos!), the problem of backing up one’s files becomes a crucial one. In times before the TimeCapsule Apple offered server iterations of Mac minis: Mac mini Core 2 duo/2.53 server, Mac mini Core 2 duo / 2.66, mid 2010 server.

Contents

1 RAIDS: Know how to back up your data on home servers
2 History of Raid
3 RAID’s Levels
4 Links

RAIDS: Know how to back up your data on home servers

While reading the specifications to them, you might have run into such words as RAID 1 or RAID 0 and wondered what it all meant.

It’s time to reveal this sacred secret of system administrators. RAID stands for Redundant Array of Independent Disks. Don’t panic, redundant doesn’t mean you have to buy HDDs by the dozen; two will do. Mac mini servers usually have dual 500 GB SATA hard drives with the 3 Gb\s speed. They are identical in capacity and rotation speed.

RAID 0 splits your data evenly between two disks. It’s kind of like storing a puzzle’s pieces not in one, but in two separate boxes. If a one box gets lost, your picture will never be complete again. Imagine that happening to your docs or prom video! Unfortunately, there’s no protection here if one of the drives faults, which they DO every now and then. But,

RAID 0 servers are quick to read and write, and both HDDs contribute to storage capacity.

RAID 1, or Mirror RAID just mirrors your data, as its moniker implies. This way, your docs, videos, and pictures are really backed up. The whole data array is copied to the other disk. If one of them faults, you can easily restore the information. But physically, you can use just one half of your storage space. If under RAID 0, you can have 500 GB x 2 = 1 TB of disk space. Alternatively, you’ll have just 500 GB of disk space under RAID 1 and your hard drive’s response will be slower.

We, at iGotOffer hope that this article will help you to make a well-educated choice when installing OS X Server on you Macs!

History of Raid

Raid was originally short for Redundant Array of Inexpensive Disks, and not the Redundant Array of Independent Disk as the term in commonly seen today. The underlying concept of Raid was first spoken of by Gus German and Ted Grunau, the two co-founders of Geac Computer Corporation (which was incorporated in 1971 and acquired by Infor Global Solutions in March 2006 for $1 billion). The creators referred to Raid as MF-100.

It was David Patterson, Randy Katz and Garth A. Gibson from the University of California however, who invented the term “RAID” in 1987. They published their paper “A Case for Redundant Arrays of Inexpensive Disks (RAID)” in June 1968 and presented it at the SIGMOD conference that year. In that paper, the authors argued that the top performing mainframe disk drives of the time could be beaten on performance by an array of the inexpensive drives that had been developed for the growing personal computer market.

Although failures would rise in proportion to the number of drives, by configuring for redundancy, the reliability of an array could far exceed that of any large single drive. We should also mention, that although not yet using that terminology, the technologies of the five levels of RAID named in the June 1988 paper were used in various products prior to the paper’s publication. These products included the following: In 1977, Norman Ken Ouchi at IBM filed a patent disclosing what was subsequently named RAID 4. Around 1983, DEC began shipping subsystem mirrored RA8X disk drives (now known as RAID 1) as part of its HSC50 subsystem. In 1986, Clark et al. at IBM filed a patent disclosing what was subsequently named RAID 5. Around 1988, the Thinking Machines’ Data Vault used error correction codes (now known as RAID 2) in an array of disk drives. A similar approach was used in the early 1960s on the IBM 353. From that time on, industry RAID manufacturers tended to interpret the acronym as standing for “Redundant Array of Independent Disks”.

RAID’s Levels

RAID 0

RAID 0 consists of striping, without mirroring or parity. The capacity of a RAID 0 volume is the sum of the capacities of the disks in the set, the same as with a spanned volume. There is no added redundancy for handling disk failures, just as with a spanned volume. Thus, failure of one disk causes the loss of the entire RAID 0 volume, with reduced possibilities of data recovery.

RAID 1

RAID 1 also consists of data mirroring, without parity or striping. Data is written identically to two drives, thereby producing a “mirrored set” of drives. Thus, any read request can be serviced by any drive in the set. If a request is broadcast to every drive in the set, it can be serviced by the drive that accesses the data first (depending on the seek time and rotational latency) which can improve performance.

RAID 2

RAID 2 consists of bit-level striping, with dedicated Hamming-code parity. All disk spindle rotation is synchronized and data is striped in such a way that each sequential bit is on a different drive. Hamming-code parity is calculated across corresponding bits and stored on at least one parity drive. This level is of historical significance only.

RAID 3

RAID 3 consists of byte-level striping with dedicated parity. All disk spindle rotation is synchronized and data is striped in such a way that each sequential byte is on a different drive. Parity is calculated across corresponding bytes and stored on a dedicated parity drive. Implementations do exist, however this level is not commonly used in practice.

RAID 4

RAID 4 consists of block-level striping with dedicated parity. This level was previously used by NetApp, but has now been largely replaced by a proprietary implementation of RAID 4 with two parity disks, called RAID-DP. The main advantage of RAID 4 over RAID 2 and 3 is I/O parallelism: in RAID 2 and 3, a single read/write I/O operation requires reading the whole group of data drives, while in RAID 4 one I/O read/write operation does not have to spread across all data drives.

RAID 5

RAID 5 consists of block-level striping with distributed parity. Unlike RAID 4, parity information is distributed among the drives, requiring all drives but one to be present to operate. Upon failure of a single drive, subsequent reads can be calculated from the distributed parity so that no data is lost. RAID 5 requires at least three disks, however. RAID 5 implementations are susceptible to system failures, because of trends regarding array rebuild time and the chance of drive failure during rebuild.

RAID 6

RAID 6 consists of block-level striping with double distributed parity. Double parity provides fault tolerance up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems, since large-capacity drives take longer to restore. RAID 6 requires a minimum of four disks. As with RAID 5, a single drive failure results in reduced performance of the entire array, until of course, the failed drive has been replaced.