Adobe Photoshop CS5 for Photographers - Martin Evening [61]
Figure 2.19 You can monitor how efficiently Photoshop is running by getting the Info panel options to display an Efficiency readout. If the efficiency reading drops below 100% this means that Photoshop is having to rely on the designated scratch disk as a virtual memory source.
Scratch disk usage and history
The scratch disk usage will vary according to how many history states you have set in the History & Cache section and also how you use Photoshop. Generally speaking, Photoshop stores a version of each history state, but it does not always store a complete version of the image for each history state. As was explained in Chapter 1, the History feature only needs to save the changes made in each image tile. So if you carry out a series of brush strokes, the history only stores changes made to the altered image tiles. For this reason, although the scratch disk usage increases as you add more history steps, in practice the usage does not increase in such large chunks, unless you were to perform a series of global filter changes.
The primary scratch disk should ideally be one that is separate to the disk that's running the operating system and Photoshop. It is no good partitioning the main disk volume and designating an empty partition as the scratch disk, because the disk drive head will simply be switching back and forth between different sectors of the same disk as it tries to do the job of running the computer system and provide scratch disk space. For optimal image editing, you ideally want the scratch disk to be a fast, separate disk volume with a minimum of 20–40 GB of free space and as free as possible of any disk fragmentation. Note also the instructions in Figure 2.20 that describe how to alter the scratch disk settings during startup.
Figure 2.20 If you hold down the keys during the startup cycle as you launch Photoshop, this reveals the Scratch Disk Preferences dialog, allowing you to configure the Scratch Disk options during startup.
Scratch disk performance
With all this reliance on the scratch disk (or multiple scratch disks) to read/write data from the RAM memory buffer, the hard disk performance of the scratch disk plays an important role in maintaining Photoshop efficiency. There is provision in Photoshop for as many as four scratch disks. Each individual scratch file created by Photoshop can be a maximum of 2 GB and Photoshop can keep writing scratch files to a scratch disk volume until it becomes full. When the primary scratch disk runs out of room to accommodate all the scratch files during a Photoshop session, it then starts writing scratch files to the secondary scratch disk, and so on. This makes for more efficient and faster disk usage. Let's now look at the important factors that affect the speed of the scratch disk.
Interface connection
Most modern Macs and PCs have IDE, ATA or SATA drives as standard. A fast internal hard disk is adequate for getting started, but for better performance, you should really install a second internal or external hard drive and have this dedicated as the primary scratch disk (make this the number one scratch disk in the Performance preferences). Your computer will most likely have the choice of a USB or FireWire connector for linking external devices. USB 2 offers a much faster connection speed than the old USB 1, while FireWire (IEEE 1394) is regarded as being faster still. With USB or FireWire you can hot swap a drive