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23.2. 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 information, see
man libblkid. This man page is provided by the libblkid-devel package.
sysfs Interface
- /sys/block/
disk/alignment_offsetor/sys/block/disk/partition/alignment_offsetNote
The file location depends on whether the disk is a physical disk (be that a local disk, local RAID, or a multipath LUN) or a virtual disk. The first file location is applicable to physical disks while the second file location is applicable to virtual disks. The reason for this is because virtio-blk will always report an alignment value for the partition. Physical disks may or may not report an alignment value. - /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:
Example 23.1. sysfs Interface
alignment_offset: 0 physical_block_size: 512 logical_block_size: 512 minimum_io_size: 512 optimal_io_size: 0
