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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 BYTESis used to derive/sys/block/disk/queue/physical_block_sizeLOGICAL BLOCKS PER PHYSICAL BLOCK EXPONENTis used to derive/sys/block/disk/queue/logical_block_sizeLOWEST ALIGNED LOGICAL BLOCK ADDRESSis 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
