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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. Anaconda, the Red Hat Enterprise Linux 7 installer, 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_offsetas the offset for the start of the first primary partition. - If
optimal_io_sizeis defined (i.e. not0), align all partitions on anoptimal_io_sizeboundary. - If
optimal_io_sizeis undefined (i.e.0),alignment_offsetis0, andminimum_io_sizeis 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 7 cannot distinguish between devices that don't provide I/O hints and those that do so withalignment_offset=0andoptimal_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.
