RAID 1 Mirroring, Your Mac and Digital Photography
I am often asked about the best types of storage solutions you should use for digital photography, especially if you should look at using RAID storage on the Mac Pro. In this series of articles, I hope to explain some uses for each of the most popular RAID types, RAID 0, RAID 1 and RAID 5. RAID is not specific to an operating system, and equally applies to Mac OS X, Windows and Linux. Whilst Mac OS X is capable of supporting RAID 1 through the RAID Utility software, only the Mac Pro provides support for more than one internal disks. You can stripe across disks connected via USB or Firewire, but the gains are minimal and it is not really worth the effort.
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 1, or Mirroring. Looking at the above definition, RAID level 1 provides greater reliability, but neither greater performance or capacity. I have previously covered RAID 0 Striping in a previous article.
Overview
RAID 1 requires a minimum of 2 disks where each disk holds the same data. Both disks are written to in parallel and both disks should be the same size and ideally the same model and manufacturer to remove any inconstancies between different drives. Therefore RAID 1 provides increased redundancy as the loss of a single disk will not cause an outage or loss of data. Replacing the faulty drive will allow you to restore the mirror set. This is typically referred to as rebuilding the RAID array, and is typically managed by either the RAID software or the RAID hardware device.
RAID 1 can be expensive, as you will be doubling the number of hard drives needed. In a typical 2 drive mirror you are only using 50% of the total available storage. This doubles the $/GB.
RAID 1 is used to increase the reliability and where the loss of a drive cannot be tolerated, such as on a desktop or server that cannot sustain any downtime. For digital photographers, this may be the boot drive on a Mac Pro, or the storage drive(s) used to store photos in a studio (however, typical backup software may be adequate.) In each case, the loss of a drive would result in loss of data and potentially lost business.
A RAID 1 mirror does not provide any increase in performance or capacity like RAID 0.
Note that RAID is sometimes use to (incorrectly) describe a type of backup. This is especially true of RAID 1 mirroring, where the increased redundancy can give a false sense of security. RAID should not be implemented with the misconception that you are providing a backup of data. As a digital photographer, it is important to maintain both local and remote copies of your images.
Advantages
- Increased reliability - A failure of a drive in a mirror will not cause data loss or loss of service
Disadvantages
- Increased cost - You are effectively doubling the storage required, doubling the cost
- Not a backup - Some people that use a mirrored drive are sometimes under the false illusion that they have provided adequate backup of their data
Recommended Solutions
Boot drive - By building a Mac, PC or Linux computer with 2 identical drives it is possible to mirror the drives. On a MAC you can use either using the software Disk Utility, or using the optional Apple RAID Card (see below) and the RAID Utility. The RAID card is recommended over a software solution as it provides a more robust and secure solution. However, you can successfully implement RAID 1 mirroring using the Disk Utility included with Mac OS X. On a PC, you will need to use either motherboard- or add-on card-based RAID (sometimes called software RAID), or a dedicated hardware RAID card. Note that a dedicated hardware RAID card can costs hundreds if not thousands of dollars, where a software RAID card can be less than a hundred dollars. This is because a software RAID card only implements the minimum needed to support RAID. On Linux it is recommended to use either a dedicated hardware RAID card or Linux software RAID (using the mdadm package). In each case, it will allow the operating system to be installed on the mirrored drives, increasing the reliability of the system drives.
Mac external storage - You can also create mirrored drives on an Mac using external firewire or USB drives. For this to be successful, the drives should be the same capacity and ideally the same model and manufacturer. The Disk Utility software allows you to create both RAID 0 striped and RAID 1 mirrored arrays. A third option of concatenated disks (sometimes called JBOD) is also available that simply concatenates the drives to form a larger drive. Note that this solution will work for any Mac with multiple firewire or USB connections.
General mirrored storage - Storage can either be additional internal drives, an external RAID enclosure or part of a NAS solution. Note that some computers may have limited internal drive capacity. For example, the Mac Pro can only hold a total of 4 internal drives. Mirroring can be an expensive solution, but it does offer a high level of data redundancy. If a number of drives to be mirrored is high (such as 3 or 4), it is recommended that a RAID 5 array is investigated, as the cost can be reduced due to the total number of drives being reduced. RAID 5 will be covered in a later article.
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 Disk Utility without the optional card, the Apple RAID card provides a more robust solution.
In Summary
Implementing RAID 1 is a great way of increasing the overall reliability of your system.
Tags: RAID 1 Mirroring
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