Choosing the Filesystem Block Size and Extent Size

Choosing the Filesystem Block Size and Extent Size

XFS allows you to choose the logical block size for each filesystem. (Physical disk blocks remain 512 bytes. EFS has a fixed block size of 512 bytes.) If you use a real-time subvolume on an XLV logical volume, you must also choose the extent size. The extent size is the amount of space that is allocated to a file every time more space needs to be allocated to it.

For XFS filesystems on disk partitions and logical volumes and for the data subvolume of filesystems on XLV volumes, the block size guidelines are:

The minimum block size is 512 bytes. Small block sizes increase allocation overhead which decreases filesystem performance, but in general, the recommended block size for filesystems under 100 MB and for filesystems with many small files is 512 bytes.
The default block size is 4096 bytes (4K). This is the recommended block size for filesystems over 100 MB.
The maximum block size is 65536 bytes (64K). Because large block sizes can waste space and lead to fragmentation, in general block sizes shouldn't be larger than 4096 bytes (4K).
For the Root filesystem on systems with separate Root and Usr filesystems, the recommended block size is 512 bytes.
For news servers, the recommended block size for the news filesystems is 2048 bytes.

Block sizes are specified in bytes in decimal (default), octal (prefixed by 0), or hexadecimal (prefixed by 0x or 0X). If the number has the suffix "k," it is multiplied by 1024. If the number has the suffix "m," it is multiplied by 1048576 (1024 * 1024).

For real-time subvolumes of XLV logical volumes, the block size is the same as the block size of the data subvolume. The guidelines for the extent size are:

The extent size must be a multiple of the block size of the data subvolume.
The minimum extent size is 4 KB.
The maximum extent size is 1 GB.
The default extent size is 64 KB.
The extent size should be matched to the application and the stripe unit of the volume elements used in the real-time subvolume.