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23.3. I/O Standards

This section describes I/O standards used by ATA and SCSI devices.

ATA

ATA devices must report appropriate information via the IDENTIFY DEVICE command. ATA devices only report I/O parameters for physical_block_size, logical_block_size, and alignment_offset. The additional I/O hints are outside the scope of the ATA Command Set.

SCSI

I/O parameters support in Red Hat Enterprise Linux 7 requires at least version 3 of the SCSI Primary Commands (SPC-3) protocol. The kernel will only send an extended inquiry (which gains access to the BLOCK LIMITS VPD page) and READ CAPACITY(16) command to devices which claim compliance with SPC-3.
The READ CAPACITY(16) command provides the block sizes and alignment offset:
  • LOGICAL BLOCK LENGTH IN BYTES is used to derive /sys/block/disk/queue/physical_block_size
  • LOGICAL BLOCKS PER PHYSICAL BLOCK EXPONENT is used to derive /sys/block/disk/queue/logical_block_size
  • LOWEST ALIGNED LOGICAL BLOCK ADDRESS is used to derive:
    • /sys/block/disk/alignment_offset
    • /sys/block/disk/partition/alignment_offset
The BLOCK LIMITS VPD page (0xb0) provides the I/O hints. It also uses OPTIMAL TRANSFER LENGTH GRANULARITY and OPTIMAL TRANSFER LENGTH to derive:
  • /sys/block/disk/queue/minimum_io_size
  • /sys/block/disk/queue/optimal_io_size
The sg3_utils package provides the sg_inq utility, which can be used to access the BLOCK LIMITS VPD page. To do so, run: 
# sg_inq -p 0xb0 disk