On a Celerra NX4, is there any way to defrag a filesystem and if so, is it something that is needed? Or is this something the array takes care of by itself?
I know we have a VNX2 and it moves data each night between two tiers, I imagine that it is intelligent and with fast cache, something like a defrag wouldn't be needed. But what about these older Celerra's?
Re: Defrag a filesystem? Is that possible or needed?
FFS tries to allocate 'logically close' blocks close. For instance, it allocates (if possible) regular file inodes in the same group as the directory holding them, and data blocks for a file in the same group as the file's inode. Alternatively, it allocates directory inodes on the less loaded group, and it forces the change of cylinder group at each megabyte of file size (to avoid fragmentation inside a cylinder group). The allocation uses geometry parameters to compute a ``closest block''.
So, there is a defrag process but, basically, you don't need to defrag since the file system code is 'tuned' to discourage fragmentation. You'll likely get a low fragmentation (2-5% of the files or so), but this will remain constant except when the partition gets very full and there are lots of file creations/deletions with extremely funny file sizes. From our experience with Celerra, EMC have yet to find a customer who has needed to defrag file systems to regain access performance.