RAID 1 (Mirroring) for Digital Photography

8 May, 2008 Storage

RAID 1 Logo 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 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.

RAID 1 Slide

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. 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.

However, 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. 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

The following recommendations are tailored towards the needs of a digital photographer.

Mac Pro boot drive - By building a Mac Pro with 2 identical drives it is possible to mirror the drives 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.

iMac external storage - You can also create mirrored drives on an iMac 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 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 also work for any Mac with multiple firewire or USB connections.

Mirrored storage - Storage can either be additional drives in a Mac Pro (the Mac Pro holds a total of 4 drives), an external RAID enclosure or part of a NAS solution. Mirroring can be expensive, 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

Apple Mac Pro 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.