Specs to Consider When Buying a RAID [u]

I need to buy a new RAID for our company server. The one we have is both too slow and too full. I started thinking about this during this year’s NAB as I was talking to a variety of storage vendors about their latest products.

NOTE: A RAID (Redundant Array of Inexpensive Drives) is a collection of hard disks that are stored in a single box, connect via a single cable and, when attached to the computer, act as though they were one very large, very fast hard drive. RAIDs are used when you need more speed or storage capacity than a single hard drive can provide.

Our server is a fairly new Mac Mini with a Thunderbolt port, so I’m looking for a Thunderbolt RAID 5. (Here’s an article that describes what the different RAID levels mean.)

Our office network includes about twelve computers, three of which are wireless. Three wired computers do audio and video editing, while the rest handle standard office and web work.

The network is wired for gigabit Ethernet, as fiber or 10-gig Ethernet is way outside my budget. What this means is that the maximum data transfer rate between the server and wired computers is limited by the gigabit Ethernet Protocol, which is about 110 MB/second after overhead. Wireless devices will be much slower, depending upon which wireless protocol they support.

The genesis of this article came when I started to think about what gear to buy.

NOTE: Your network switch will often determine how efficient your network is. Low-cost switches often can’t support transferring data at full speed to all connected computers at the same time. For this reason, I upgraded to a Cisco SMB switch. It costs more, but avoids bottlenecks.

FACTORS TO CONSIDER

In thinking about this, I realized that there are five main factors to consider when buying a RAID:

• Storage capacity
• Connection Protocol
• Data Transfer Rate
• IOPS
• The type of RAID controller

The importance of each of these factors changes depending upon your needs. So, let me explain what they are and when you need to consider them for your own system.

STORAGE CAPACITY

The storage capacity of a RAID (or any hard disk) is measured in either Gigabytes or Terabytes. A Gigabyte is 1,024 Megabytes, while a Terabyte is 1,024 Gigabytes.

NOTE: In an effort to make storage numbers more accessible, many technology marketing departments describe a gigabyte as 1,000 MB, or a terabyte as 1,000 GB. However, hard disks don’t read marketing literature. After formatting, RAIDs will always store less data than printed on the packaging.

We are all familiar with picking a hard disk based on storage capacity. If we are storing small files, a smaller capacity is fine. If we are storing large files, more space will be necessary.

As you are deciding what size RAID to buy, keep in mind the adage: “It is impossible to buy a hard disk that is either too big or too fast.” Always buy a bit more than you need.

CONNECTION PROTOCOL

In the past, we had two choices on how to connect a hard drive to our computer:

• USB 2
• FireWire

Those protocols were good for the time, but either was very fast. In fact, the protocol was slower than the internal speed of a hard disk. (FireWire 800, for example, transfers data at about 85 MB/sec.)

Now, we have several additional new choices:

1. USB 3
2. Thunderbolt
3. Thunderbolt 2
4. Mini-SAS
5. eSATA

For Mac users, the last two options: mini-SAS and eSATA require PCIe cards or conversion boxes. While the protocols themselves are excellent, if you are starting fresh use Thunderbolt because it is easier to connect and as fast, or faster, than mini-SAS or eSATA.

I’ve been told that USB is optimized for smaller files – think office files – while Thunderbolt is optimized for larger files – think media files. When it comes to speed, these new protocols are EXTREMELY fast:

• USB 3 has a theoretical speed of 640 MB/second
• Thunderbolt 1 has a theoretical speed of 1.1 GB/second
• Thunderbolt 2 has a theoretical speed of 2.2 GB/second

However, as you’ll see in the next section, theoretical speed and actual speed are radically different.

DATA TRANSFER RATE

The data transfer rate is the speed that data travels between the computer and the RAID (or hard disk). For RAIDs and other storage, we measure this in MB/second; higher numbers indicate faster performance.

What you need to understand about standard hard drives – also called “spinning media” – is that a single hard drive can only transfer data at about 120 MB/second. (SSD, or Flash drives, are much faster and we’ll talk about them in a minute.)

This means that if you need speeds faster than 120 MB/second, you need to group multiple hard drives to work together. This is what a RAID does – it groups a bunch of standard hard disks together so they can transfer data faster.

Just to help you think about this, here are some data transfer speeds of different codecs (actual data transfer rates will vary with image size and frame rate):

• AVCHD is about 3 MB/sec
• ProRes 422 Proxy is about 5.5 MB/sec; this is the format FCP X uses for proxy media
• ProRes 422 is about 18 MB/sec; this is the format FCP X uses to “optimize” media
• ProRes 422 HQ is about 27.5 MB/sec
• ProRes 4444 is about 41 MB/sec without including the alpha channel
• RAW image formats easily exceed 100 MB/sec, depending upon image size and frame rate

For multicam editing, multiply the speed of the codec you are using by the number of cameras you are editing.

Data transfer rate is the most important spec we need to consider for direct-attached RAIDS and drives. However, if we are attaching a RAID to a server, the data transfer rate is determined by the network protocol, in my case gigabit Ethernet, not the speed of the RAID. A fast data transfer rate is important, but not critical.

A SIDE NOTE ON SSD DRIVES

SSD stands for “Solid State Drive.” It takes a bunch of RAM and makes it look like a hard disk to the computer. SSDs provide all the speed of RAM with the permanent memory of spinning media.

The good news is that SSDs are very, VERY fast. The bad news is that they are very expensive and don’t store as much as spinning media.

Depending upon which controller the SSD uses and the type of NAND-based flash memory, SSD drives can attain speeds of more then 1.0 GB per second when playing back a single files sequentially. This is fast enough to fully fill a Thunderbolt 1 connection. However, SSD speeds slow down dramatically when performing random reads and writes, which is what a server requires.

In general, SSDs are an excellent choice for boot drives and, if you can afford them, for media drives that are direct attached. However, for small businesses, a standard hard drive is a better option for server storage because most of that speed is lost when transferring data over the network.

Here’s an excellent article that compares SSD drives with standard hard drives.

IOPS

While the data transfer rate is critical for direct-attached RAIDs, when we are attaching a RAID to a server, a different measurement becomes more important: IOPS (pronounced: “eye opps”). This is the number of Input / Output oPerations per Second the RAID can perform.

With a server, multiple users are accessing different files on the same RAID at the same time. IOPs measure how quickly the RAID can respond to all these different requests.

Since the overall data transfer rate is determined by the network – which is FAR slower than the native data transfer rate of the RAID – we need to concentrate on which RAID can process the greatest number of requests in the least amount of time within the budget that we have to work with.

NOTE: As you might expect, high-performance storage with high IOPS to meet the needs of hundreds of users costs in the tens-of-thousands of dollar range, and fills entire equipment racks with drives. While providing vast performance and storage, they are beyond the budget of most smaller shops, like mine. As with all things, we need to balance performance against budget.

Calculating IOPS involves some tricky math and varies depending upon the RAID level you are using. (Do a Google search for “Calculate IOPS” and you’ll see what I mean.) However, when you are buying a RAID for a server, check it’s IOPS rating. The higher the IOPS rating, the better the RAID will perform when multiple users are accessing the RAID at the same time.

RAID CONTROLLER

There are two types of RAID controllers: hardware-based and software-based.

• Hardware RAID controllers are REALLY fast, however, they are not very flexible. RAIDS from MaxxDigital, G-Technology, CalDigit, and Akitio (among others) all use hardware RAID controllers.
• Software RAID controllers are very flexible, but not very fast. Drobo is best known for its software RAID controller which allows expanding storage and repartitioning a RAID without destroying the data stored on it.

When performance is important, look for a hardware RAID controller. When flexibility is more important, a software RAID controller may be a better choice.

SUMMARY

The media storage industry is in the process of transitioning from older protocols to Thunderbolt 1 and 2. I saw this in all the announcements that were made at NAB in April of this year. Supporting Thunderbolt, or USB 3, means that storage can be attached to any Mac without needing a PCIe card or converter box.

And, for users with a deep investment in PCIe cards, expansion options were offered from a wide variety of vendors, including ATTO, Sonnet, mLogic and Akitio.

However, most of these new products won’t be shipping for a while. So this gives me time to do my research and figure out which drives makes the most sense for what I need.

Ultimately, I will be buying two RAIDs: one for the server and one for high-performance direct-attached editing. Given what I’ve learned in researching this article, they won’t be the same product, because they don’t do the same job.

I’ll let you know what I decide. In the meantime, I’m always interested in your thoughts.

UPDATE – April 21, 2014

After I published this article, I realized that I forgot to include a link to other articles I’ve written on storage. There is a wealth of information here that can save you a ton of headaches: Storage Basics – Collected Articles