RAID LEVELS DEFINED
RAID (Redundant Array of Independent Disks) is a way of storing data on two or more drives in combination for Failure Tolerance, Redundancy and/or Performance gains. By placing data on multiple disks configured as a single volume I/O (input/output) operations can overlap improving performance, or by mirroring multiple disks thus increasing the mean time between failures providing some Failure Tolerance.
Please understand that no storage is 100% safe. Any type of RAID can fail at any time. The only true data security comes from multiple copies of your data, preferably in separate physical locations. Keep your data backed up at all times.
RAIDs are not for everyone, in fact most people should avoid them and stick with a backed-up JBOD system managed by software. In some instances using a RAID actually puts your data more at risk. (Striped RAID arrays lose their data if one member fails. When you combine multiple drives in an array you increase the chance of a single drive failing by the number of drives included, i.e., a four drive RAID 0 array would have four times the chance of a drive failing) Sometimes this can't be helped such as Video Editing where the only way to achieve the needed data throughput speed is striping the disks together to gain their combined speeds and volume. In this case you'll want to have a good Backup Routine in place that backs up to an array of equal size every few hours or so. Should you fail an array you can keep working by using the backup copy as your main data storage while repairs are made to your first array. This is why we recommend that you have a second backup. You never want to work on your only viable copy, it's like walking a tightrope without a net.
NOTE: Windows 32 bit Operating Systems can only access volume sizes of 2TB or smaller. This means that if you have a RAID Controller that creates a RAID Volume larger than 2-TB, Disk Management will not be able to initialize it. There are a couple of workarounds for this. RAIDs build in Disk Management can be of any size. Windows 64 bit based OS's don't have this issue.
COMMON RAID TYPES
- Striped Disk Array - Without Fault Tolerance
- Combines the speed and volume of multiple drives
- This technique has striping but no redundancy of data. It offers the best performance but no Fault Tolerance. By far the most common means of attaining the maximum data throughput speeds needed for HD Video editing.
- RAID 0 combines the individual speed and volume of each member to produce screaming fast reads and writes. If the sizes of the drive members are different, the smallest member will limit the overall size of the RAID Group.
- RAID 0 is usually configured within the Operating System and requires no special hardware
- If one drive fails or is damaged all data in the array is lost
- Mirrored Disk Array - Creates drives identical to one another
- RAID 1 consists of at least two drives that duplicate the storage of data
- Read performance is improved since either disk can be read at the same time but write performance is the same as a single disk
- This RAID is mostly useful for server applications where 100% uptime is required. If one disk fails the other has a real-time version of the data
- If the data is accidentally erased or corrupted it's gone from both drives. RAID 1 does not protect you from software problems, file corruptions or human error. The only protection it provides is for a hard drive failure.
- RAID 1 should NEVER be used as a backup
- A Mirror of Stripes: A pair of identical RAID 0s
- Two RAID 0 Stripes are created (each with at least two drives), then a RAID 1 Mirror is created over them
- This provides better performance than a RAID 1 but at a higher cost
- Mainly used in servers that need a fast mirror
- Striped Array with a Dedicated Parity Drive
- A RAID 3 array moves data in large chunks instead of at the bit level like a RAID 5
- Parity data is stored on a single drive which allows a rebuild after a drive failure while maintaining the stored data
- This type of a RAID is most commonly used for large single files such as video or audio
- Distributed or Shared Parity
- Sometimes called Rotating or Revolving Parity
- Parity refers to the technique of checking whether data has been lost or written over at the bit level when it's moved from one place in storage to another. Should you fail a drive it can be removed and replaced and the data on the missing drive recreated on the fly by using the parity data as a reference.
- RAID 5s require intense CPU time to manage the parity information. For this reason it's usually best to use a Hardware based RAID Controller that has a dedicated onboard CPU and RAM specifically for this task to avoid taxing system resources.
- Due to the parity the storage capacity of one drive is lost to the RAID
- Intolerant of data corruption, corruption can fail this RAID type
- Sriped RAID 5 - Two or more RAID 5 volumes are striped together to form a RAID 0
- This provides the combined storage volume and speeds with maintained Failure Tolerance
- A drive can be failed in either RAID 5 set and the data will be retained allowing the array to be rebuilt
- Intolerant of data corruption, corruption can fail this RAID type
- Distributed Parity
- Same thing as a RAID 5 except it uses two sets of revolving parity and can therefor fail two drives
- Slightly safer than a RAID 5 array at slightly higher cost
- Intolerant of data corruption, corruption can fail this RAID type
- Just a Bunch Of Disks
- This is where all of your mounted hard drives are independent from one another. Each drive is it's own entity
- Very common for photographers. Drives can be managed by the job, location, customer or just chronologically.
- Each drive can be named for easy reference and should have at least one backup.
- This is by far the simplest and easiest to manage system of storage.
NOTE: Zero out Data - During testing we have found many benefits for erasing new drives and writing a pass of zeros to them before formatting them into an array or storing data on them. This will allow the new drives to test and repair or map as needed every bit on the platter. It makes the drives faster and a little more stable when used as a RAID. This is accomplished in a Mac by using the "ERASE" function in Disk Utility and choosing the "ZERO OUT DATA" option found under "SECURITY OPTIONS". There are many utilities written for the PC platform that perform the same function such as Western Digitals free "
Data Lifeguard Diagnostics" software. Make sure you know what drive you're working with, it's all too easy to erase the wrong drive.
NOTE: Drives must never be filled to their capacity. If drives become too full you stand the risk of overwriting the index or causing data corruption. Keep your data volume under 70% of the drive capacity for photos and OSs', stop at 80% for video.
