Conventional wisdom regarding Linux filesystems suggests that file defragmentation is not a necessary task. While this is true in general, file fragmentation isn't something we should allow to spiral out of control. PostgreSQL storage files are limited to 1 GB in size, yet we configured XFS to pre-allocate no more than 1 MB at a time.
This introduces the potential for data fragmentation on OLTP systems or any database cluster where several tables experience high turnover. To prevent this from adversely affecting sequential scans, and to promote good filesystem health in general, we need to track and potentially correct overly fragmented files.
XFS provides two tools suited to this activity. The first is xfs_db, which provides information about an XFS filesystem. The second is xfs_fsr, which allows us to defragment XFS while it is still mounted and active. This recipe will cover...
