The acronym for RAID can often be misleading as it has had multiple meanings over the years. RAID originally stood for a redundant array of inexpensive disks. The acronym RAID is now also known as a redundant array of independent disks as not all RAID disks are inexpensive. RAID refers to a hard drive storage mechanism using multiple hard drives to share or replicate data among the drives. In some cases this can mean having data that is written to a single logical drive stored on multiple drives so there is redundancy of the data or RAID can be used to maximize throughput to drives by aggregating possible speeds of RAID member drives. A key advantage of RAID is the ability to combine drives into an array with more capacity, reliability, speed, or a combination of these, than was affordably available in a single device.

Through the remainder of this article we will be looking at different types of RAID and what each can do. But before we look into RAIDs let’s look at a JBOD. In a JBOD (Just a Bunch of Disks) which is also often called a concatanated RAID in OS X, you can use multiple drives to merge data into one volume. You can take 2 drives of 2 Terabytes each and 4 drives of 1 Terabyte and merge them into one volume of 8 Terabytes. In this scenario you would end up with no fault tolerance in your environment but you would be able to take use of low cost drives, such as LaCies to create a single volume. However, if any of the drives in a JBOD fail the full volume will fail. This leads us to use this type of situation primarily with volatile situations such as a disk-to-disk backup solution.

A RAID0 is similar to a JBOD; however RAID0 requires all drives in a RAID0 array to be identical in size. Provided the drives are the same in size, RAID0 offers the fastest speeds available in a RAID. These are often used for high definition video editing and volumes housing database volumes requiring a lot of speed. RAID0 does not offer any redundancy of data. If one member in the array fails, just as with a JBOD then the volume will fail as well. However, RAID0 is fast and an inexpensive way to get large amounts of fast storage.

RAID1 is often known as a mirror. In RAID1, all data written to one disk is then duplicated onto a second disk of an identical size. In a mirror, if one member of the set of disks were to fail then the disk would continue to be accessible for read/write operations. RAID1 offers amongst the best protection to data loss available to RAID scenarios, but at the highest cost. For every byte of data stored on a RAID1 volume there must be an equal byte used for redundancy. As high end disks have become more and more expensive the development of more complex RAID strategies helps to maximize our ability to make use of a variety of solutions.

In RAID 3 you would end up with one member of each array as a static parity drive. This drive will store a stripe of information about each other drive and if one of them crashes will create itself in that drives image. The parity also causes a slight loss of speed over if it was a large RAID0 volume. RAID0 in the truest sense of the word (no parity) would net you 100% of the usable space.

Parity information is stored striped across all of the drives in RAID5, not just one. In RAID3, parity information is stored on a dedicated parity drive. But even in RAID3 you shouldn’t be able to make the smallest drive the hot swap. In fact you can only typically build a RAID0 out of drives of different sizes (which isn’t much of a RAID but more of a JBOD) unless you slim all drives down to the smallest drive size manually. Thus, a RAID5 + Hot Spare array of a 5 40GB drives would end up being a RAID 5 volume of 4 40GB drives. If you pull one for hot spare you would end up with an 80GB volume. This nets a 33% loss of space. If all 4 drives were in the RAID then you would get a RAID5 volume of 120GB, netting a 25% loss. If 5 drives at 40GB were in the RAID you would end up with a 160GB volume; thus resulting in only a 20% loss. And so on. The parity information is stored on all drives so any single drive can go down and the contents of the RAID will be rebuilt based on the parity stored on each of the drives.

RAID6 offers even more redundancy by writing two stripes of parity information to each member of the array. This allows for two drives in the RAID to crash without loosing data. RAID6 comes with more cost than most other RAIDs, both in RAID hardware and hard drives, and so is used much more rarely.

This is a great time to be a part of the post-production industry, and one of the most exciting aspects of production-house technology is Apple’s new xsan (storage area network). Xsan can have volumes up to 2 petabytes, operates up to 20 times faster than typical network storage and allows editing HD video stored on a central server.
The savings in capture cards and storage can be staggering as Production companies grow. With a SAN you can also add stock footage, sound effects libraries and reels with the space you’ve saved. The savings in time can far outweigh the cost of equipment.
Multiple editors often work on the same project with an assistant or an intern copying files to each system. An assistant editor or intern can lay off footage onto the Storage Area Network (SAN). This allows the assistant or intern to save the company the cost of purchasing extra drives for duplicate media and have more free time to build sound effects libraries or run errands.
Imagine you have 10 Final Cut stations set up to do uncompressed SD and HD. Each suite has two video tape recorders, a really good capture card and a RAID. This can be very costly. With xSAN you can still have 10 Final Cut stations. Instead of having 20 tape recorders, 10 really good capture cards, and 10 RAIDS you end up with two video tape recorders, one really good capture card, nine decent capture cards for monitoring, and an appropriate amount of space on the SAN for storage.
A certain amount of network bandwidth is used controlling file access. In xSAN, a metadata controller (MDC) frees up the Fiber Channel by controlling the read and write access for the files. Xsan uses a separate Ethernet network for metadata. In most cases, this means adding a second network, one for metadata and one for general IP networking, such as web and email access.
You can connect up to 64 systems to one Xsan. The Metadata Controller counts as a node as does the backup metadata controller if one is present (backup metadata controllers help to increase uptime). This allows for 62 possible stations accessing the Xsan.
When using an XSAN it is possible to expand volumes on the server without reformatting the drives. This is possible because of virtualization. Virtualization is a term used to describe the technique of managing and presenting storage devices and resources functionally, regardless of their physical layout or location.
A quota is the amount of space a user can take up on a SAN. A soft quota allows users to continue saving files and warns when they exceed their limit. When they reach the hard quota they are no longer able to save files until the xSAN administrator gives them more space or they remove some files from the SAN.
Implementing an Xsan system typically forces users into a centralized directory structure, which can be hosted on the MDC. You can bind Xsan nodes to Apple’s Open Directory or Windows Active Directory. Directory Services can help secure files and avoid network probelms. Directory Services can also give administrators the ability to implement stringent desktop policies such as those required by the MPAA.
XSan can do a lot and will change the way many Final Cut editors work. If you have any questions about Xsan systems or integration, just call Three18 at 310-581-9500. Work smarter, not harder.