This question is a little esoteric. I decided to try improving mhonarc's archiving speed with one of those whiz-bang solid state drives from Intel. Unfortunately, they are pretty low capacity and I can't fit all the data. So I decided to go with a hybrid strategy; all new writes go to the SSD, but tons of files still exist on the rotating rust. The way to do this is a union mount, and Linux has tons implementation floating around. I chose one called aufs. Unfortunately, aufs is not that great when listing directory contents. I *think* mhonarc is doing a listdir operation as a safety check to avoid clobbering existing message files. (I tried to find out via strace, but managed instead to confuse myself). Is this actually true, and if so, can the directory listing be reasonably turned off?
$ time ls -U /mnt/rotating-rust | wc -l # cached 1382438 real 0m2.224s user 0m1.600s sys 0m0.900s $ time ls -U /mnt/whizzy-ssd | wc -l 10099 real 0m0.013s user 0m0.000s sys 0m0.010s $ time ls -U /mnt/the-unholy-union | wc -l 1391390 real 1m10.115s user 0m0.540s sys 1m9.430s
