If your disks are >1TB with XFS then try mount -o inode64 This has the effect of sequential writes into the same directory being localised next to each other (within the same allocation group). When you skip to the next directory you will probably get a different allocation group. Without this, the behaviour is to (a) stick all the inodes in the first allocation group, and (b) to stick every file into a random allocation group, regardless of the parent directory
