The block size is file system specific. I just know that xfs can use block size greater than 4k in linux . Block size is limited by pagesize for any filesystem, xfs included. From the man pages: -b Block size options. This option specifies the fundamental block size of the filesystem. The valid suboptions are: log=value and size=value; only one can be supplied. The block size is specified either as a base two logarithm value with log=, or in bytes with size=. The default value is 4096 bytes (4 KiB), the mini mum is 512, and the maximum is 65536 (64 KiB). XFS on Linux currently only supports pagesize or smaller blocks. From a test: With 4k: # mkfs.xfs -f -b size=4k /dev/cciss/c0d0p7 meta-data=/dev/cciss/c0d0p7 isize=256 agcount=8, agsize=152744 blks = sectsz=512 data = bsize=4096 blocks=1221952, imaxpct=25 = sunit=0 swidth=0 blks, unwritten=1 naming =version 2 bsize=4096 log =internal log bsize=4096 blocks=2560, version=1 = sectsz=512 sunit=0 blks realtime =none extsz=65536 blocks=0, rtextents=0 breonldbls05:~ # mount -t xfs /dev/cciss/c0d0p7 prova With 64k: # mkfs.xfs -f -b size=64k /dev/cciss/c0d0p7 meta-data=/dev/cciss/c0d0p7 isize=256 agcount=8, agsize=9546 blks = sectsz=512 data = bsize=65536 blocks=76368, imaxpct=25 = sunit=0 swidth=0 blks, unwritten=1 naming =version 2 bsize=65536 log =internal log bsize=65536 blocks=1000, version=1 = sectsz=512 sunit=0 blks realtime =none extsz=65536 blocks=0, rtextents=0 breonldbls05:~ # mount -t xfs /dev/cciss/c0d0p7 prova mount: Function not implemented Regards Fabrizio