mount /images on one disk and /images/dev on another disk
For fog, this is not a good idea. The issue comes on image upload. FOG will capture an image to /images/dev directory because that is the only directory that has a share level of read-write. At the end of the upload process the FOS Linux engine connects to the fog server over ftp and issues a
mv command to move the files from
/images/<image_name>. The problem is this: If the image is on the same physical disk the only thing that is changed is the file pointers no data is copied anywhere. The raw data files stay on the hard drive where they were uploaded only the directory file pointers are changed. Now if /images/dev are on one disk and /images are on a different disk, the ftp mv command has to copy all the data from one disk to the other disk. This takes time copying a multi-GB file from one disk to the next. Will it work, yes. Will your upload take a long time, yes.
There is no advantage to splitting /images and /images/dev into two disks, but there are a few disadvantages.