RAID 0 (Striping) for Digital Photography
I am often asked about the best types of storage a digital photographer should use, especially if they should use RAID storage. In this series of articles, I hope to explain some uses for each of the most popular RAID types, especially in the context of digital photography.
RAID (Redundant Array of Inexpensive Disks) is the use of 2 or more disks to achieve greater reliability, greater performance or greater capacity.
In this article, I will be describing RAID level 0, or Striping. Looking at the above definition, RAID level 0 provides greater performance and to a lesser extent greater capacity.
Overview
RAID level 0 requires a minimum of 2 disks to create the RAID array. Each drive (or partition, as a RAID 0 array can be made across like-sized partitions) needs to be the same size, and it is recommended that the drives are the same make and model to remove any inconstancies between different drives. The data is striped across all disks in the array (defined as the stripe set). Data is spread evening across the disks as shown in the diagram above, therefore providing increased performance as parallel disk reads and writes are quicker than accessing a single drive. This is because the hardware I/O speed is quicker than disk access performance, so the computer can read data from 2 or more disks a lot faster than if it was reading the same data from a single drive.
The two main metrics used when describing disk performance are access time and transfer rate. The access time (expressed in milliseconds) is the time it takes for the drive to move the disk heads from rest to the correct location on the disk. As the data can be accessed quicker at the edge of the disk, and slower near the center (as the heads are parked on the edge when the disk is not being accessed), the access time is expressed as an average across the range of the disk heads. A RAID 0 array will have a slightly slower access time than a single drive; however, access times are not noticeable to end-users. The other metric, transfer rate, is more noticeable to end-users. A typical drive (such as a modern SATA drive) can achieve a maximum transfer rate of about 80MB/s. A 2 drive RAID 0 array will achieve about 140MB/s, and a 3 disk RAID 0 array about 180MB/s. As you can see, there is a trailing off of performance as more disks are added to the RAID 0 array. This is because the bottleneck moves from the disk to the underlying I/O sub-system and disk controller. The increased transfer rate explains why RAID 0 is popular when the requirement is to read (and to write) large volumes of data.
However, RAID 0 does reduce the overall reliability, as a failure of a single device in the array will cause the stripe set to be inoperable. Replacing the faulty drive will not allow you to recover the striped set, as the data on the faulty drive cannot be recovered from the data on the other drives (unlike a RAID 5 array for example.)
Note that RAID is sometimes use to (incorrectly) describe a type of backup. RAID should not be implemented with the misconception that you are providing a backup of data.
Advantages
- Increased performance - The transfer rate is increased allowing applications to read and write data faster
- Increased capacity of a single drive - Smaller physical drives are presented as single logical disk, giving the impression of increased capacity
Disadvantages
- Reduced reliability - The failure of a single drive will result in the stripe set being inoperable
- Increased physical requirements - Increasing the number of disks also increases the required number of drive bays, power and data connectors and increased cooling requirements
Recommended Solutions
The following recommendations are tailored towards the needs of a digital photographer. RAID 0 does have wider uses, such as with video editing.
Faster System Drive - Due to the increased performance of both reads and writes, using a striped array for the system drive can dramatically improve the responsiveness and overall performance of your system. However, you will need to ensure that a backup is made of the array as the reliability of a striped array is reduced. Note that you should really be making a backup of your system drive irrespective of if a striped array is used. My preference is to use SuperDuper! to create a bootable backup of your drive on a daily basis to a locally connected firewire (or USB) drive.
Photoshop Scratch Space - The Photoshop scratch space can take advantage of the increased performance of both reads and writes. Reliability is less of an issue, as the scratch space is only for temporary storage.
Mac Pro - Apple RAID Card
If you are looking at the Mac Pro, Apple have included the option of the Apple RAID card. The card includes 256MB of RAID cache, a 72-hour cache-protecting battery, and support for hardware RAID levels 0, 1, 5, and 0+1. The card works with both SATA and SAS drives and requires a spare PCI slot. As the RAID is managed in hardware on the card, minimal system resources are consumed managing the RAID array. Even though Apple supports software RAID levels 0 and 1 using the RAID Utility without the optional card, the Apple RAID card provides a more robust solution.
In Summary
Implementing RAID 0 is a great way of increasing the overall performance of your system. The increased read and write performance can also improve Photoshop’s performance. However, you should also be aware of the reduced reliability of a RAID 0 array, and make allowances with a thorough backup strategy.
Tags: RAID 0 Striping, Striped Disk
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