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18.2. Userspace Access

Always take care to use properly aligned and sized I/O. This is especially important for Direct I/O access. Direct I/O should be aligned on a logical_block_size boundary, and in multiples of the logical_block_size.

With native 4K devices (i.e. logical_block_size is 4K) it is now critical that applications perform direct I/O in multiples of the device's logical_block_size. This means that applications will fail with native 4k devices that perform 512-byte aligned I/O rather than 4k-aligned I/O.

To avoid this, an application should consult the I/O parameters of a device to ensure it is using the proper I/O alignment and size. As mentioned earlier, I/O parameters are exposed through the both sysfs and block device ioctl interfaces.

For more details, refer to man libblkid. This man page is provided by the libblkid-devel package.

sysfs Interface

/sys/block/disk/alignment_offset
/sys/block/disk/partition/alignment_offset
/sys/block/disk/queue/physical_block_size
/sys/block/disk/queue/logical_block_size
/sys/block/disk/queue/minimum_io_size
/sys/block/disk/queue/optimal_io_size

The kernel will still export these sysfs attributes for "legacy" devices that do not provide I/O parameters information, for example:

alignment_offset:    0
physical_block_size: 512
logical_block_size:  512
minimum_io_size:     512
optimal_io_size:     0

Block Device ioctls

BLKALIGNOFF: alignment_offset
BLKPBSZGET: physical_block_size
BLKSSZGET: logical_block_size
BLKIOMIN: minimum_io_size
BLKIOOPT: optimal_io_size