23.6. Partition and File System Tools
23.6. Partition and File System Tools This section describes how different partition and file system management tools interact with a device's I/O parameters. util-linux-ng's libblkid and fdisk The libblkid library provided with the util-linux-ng package includes a programmatic API to access a device's I/O parameters. libblkid allows applications, especially those that use Direct I/O, to properly size their I/O requests. The fdisk utility from util-linux-ng uses libblkid to determine the I/O parameters of a device for optimal placement of all partitions. The fdisk utility will align all partitions on a 1MB boundary. parted and libparted The libparted library from parted also uses the I/O parameters API of libblkid . The Red Hat Enterprise Linux 6 installer ( Anaconda ) uses libparted , which means that all partitions created by either the installer or parted will be properly aligned. For all partitions created on a device that does not appear to provide I/O parameters, the default alignment will be 1MB. The heuristics parted uses are as follows: Always use the reported alignment_offset as the offset for the start of the first primary partition. If optimal_io_size is defined (i.e. not 0 ), align all partitions on an optimal_io_size boundary. If optimal_io_size is undefined (i.e. 0 ), alignment_offset is 0 , and minimum_io_size is a power of 2, use a 1MB default alignment. This is the catch-all for "legacy" devices which don't appear to provide I/O hints. As such, by default all partitions will be aligned on a 1MB boundary. Note Red Hat Enterprise Linux 6 cannot distinguish between devices that don't provide I/O hints and those that do so with alignment_offset=0 and optimal_io_size=0 . Such a device might be a single SAS 4K device; as such, at worst 1MB of space is lost at the start of the disk. File System tools The different mkfs. filesystem utilities have also been enhanced to consume a device's I/O parameters. These utilities will not allow a file system to be formatted to use a block size smaller than the logical_block_size of the underlying storage device. Except for mkfs.gfs2 , all other mkfs. filesystem utilities also use the I/O hints to layout on-disk data structure and data areas relative to the minimum_io_size and optimal_io_size of the underlying storage device. This allows file systems to be optimally formatted for various RAID (striped) layouts.