Fragmentation
Othere systems which use roll-up write techniques to optimise write performance see this sort of effect. NetApp OnTap will do it if you run to high space utilisation levels as the available free space gets fragmented and it is more difficult to do efficient writes as there are few "clean" areas to do the most efficient writes. The worst access patterns are those with lots of small, random updates (typical of a database) - if the workload is largely sequential writes then it is much less of an issue. However, given that the random read speed of these tjhings is a lot better than real disks, decent algorithms which avoid defragmentation could be implemented a lot more efficiently than with real disks.