What is the I/O Safety Buffer?
By Orren Merton (excerpted from _Logic Pro 8 Power_ by Orren Merton and Kevin Anker. ©2009 Course Technology PTR, all rights reserved)The I/O Safety Buffer, when engaged, will add an extra I/O buffer after the regular I/O buffer. This will, in effect, double the latency of the I/O buffer size you have set. So if you have your I/O buffer size set to 64 samples, for example, engaging the I/O Safety Buffer will basically increase the latency to the same amount as if you had set the I/O buffer size to 128 samples.
Enabling the I/O Safety Buffer will also reserve an entire CPU or CPU core on Macs with more than a single processor, just for recording. This means that if you have two core machine, such as a MacBook, MacBook Pro, iMac, Dual G5, and so on, engaging the I/O Safety Buffer will reserve nearly half of your available processing power to ensure a smooth recording. If you have a Mac Pro with four or eight cores, it will reserve either 25 percent or 12.5 percent of your available processing power.
As you can see, this makes the I/O Safety Buffer quite a mixed blessing. Unless you have a Mac Pro, the I/O Safety Buffer will reserve so much processing power as to make it nearly useless. Even if you do have a Mac Pro, you will get better performance by simply setting the I/O buffer size higher and keeping the I/O Safety Buffer off. However, if you do have a Mac Pro, you need to record at the lowest latency you can but you are getting crackles and pops in the recorded audio, and your Logic project currently takes up very little processing power, the I/O Safety Buffer may be perfect.