The smallest addressable block of data in Data ONTAP is 4k. However, all data is written to volumes in 256k chunks. When data block which is bigger than 256k comes in, filer searches for contiguous 256k of free space in the file system. If it’s found, data block is written into it, if not then filer splits the data block and puts it in several places. It’s called fragmentation and is familiar to everyone from the times, when FAT files ystems were in use. It’s not a big issue in modern file systems, like NTFS or WAFL, but defragmentation can help to solve performance problems in some situations.
In mostly random read/write environments (which is quite common these days) fragmentation has no impact on performance. If you write or read data from random places of the hard drive it doesn’t matter if this data is random or sequential on the physical media. NetApp recommends to consider defragmentation for the applications with sequential read type of workload:
- Online transaction processing databases that perform large table scans
- E-mail systems that use database storage with verification processes
- Host-side backup of LUNs
Reallocation process uses thresholds values to represent the file system layout optimization level, where 4 is normal and everything bigger than 10 is not optimal.
To check the level of optimization for particular volume use:
> reallocate measure –o /vol/vol_name
If you decide to run reallocate on the volume, run:
> reallocate start –f /vol/vol_name
There are certain considerations if you’re using snapshots or deduplication on volumes. There is a “-p” option, to prevent inflating snapshots during reallocate. And from version 8.1 Data ONTAP also supports reallocation of deduplicated volumes. Consult official documentation for additional information.
Further reading:
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