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Constellation.2 SAS Product Manual, Rev. H 47
10.7 Idle Read After Write
Idle Read After Write (IRAW) utilizes idle time to verify the integrity of recently written data. During idle periods,
no active system requests, the drive reads recently written data from the media and compares it to valid write
command data resident in the drives data buffer. Any sectors that fail the comparison result in the invocation of
a rewrite and auto-reallocation process. The process attempts to rewrite the data to the original location. If a
verification of this rewrite fails, the sector is re-mapped to a spare location.
10.8 Protection Information (PI)
Protection Information is intended as a standardized approach to system level LRC traditionally provided by
systems using 520 byte formatted LBAs. Drives formatted with PI information provide the same, common LBA
count (i.e. same capacity point) as non-PI formatted drives. Sequential performance of a PI drive will be
reduced by approximately 1.56% due to the extra overhead of PI being transferred from the media that is not
calculated as part of the data transferred to the host. To determine the full transfer rate of a PI drive, transfers
should be calculated by adding the 8 extra bytes of PI to the transferred LBA length, i.e. 512 + 8 = 520. PI for-
matted drives are physically formatted to 520 byte sectors that store 512 bytes of customer data with 8 bytes of
Protection Information appended to it. The advantage of PI is that the Protection Information bits can be man-
aged at the HBA and HBA driver level. Allowing a system that typically does not support 520 LBA formats to
integrate this level of protection.
Protection Information is valid with any supported LBA size. 512 LBA size is used here as common example.
10.8.1 Levels of PI
There are 4 types of Protection Information.
Type 0 - Describes a drive that is not formatted with PI information bytes. This allows for legacy support in non-
PI systems.
Type 1 - Provides support of PI protection using 10 and 16 byte commands. The RDPROTECT and WRTPRO-
TECT bits allow for checking control through the CDB. Eight bytes of Protection Information are transmitted at
LBA boundaries across the interface if RDPROTECT and WRTPROTECT bits are nonzero values. Type 1
does not allow the use of 32 byte commands.
Type 2 - Provides checking control and additional expected fields within the 32 byte CDBs. Eight bytes of Pro-
tection Information are transmitted at LBA boundaries across the interface if RDPROTECT and WRTPRO-
TECT bits are nonzero values. Type 2 does allow the use of 10 and 16 byte commands with zero values in the
RDPROTECT and WRTPROTECT fields. The drive will generate 8 bytes (e.g.0xFFFF) 8 bytes of Protection
Information to be stored on the media, but the 8 bytes will not be transferred to the host during a read com-
mand.
Type 3 - Seagate products do not support Type 3.
10.8.2 Setting and determining the current Type Level
A drive is initialized to a type of PI by using the format command on a PI capable drive. Once a drive is format-
ted to a PI Type, it may be queried by a Read Capacity (16) command to report the PI type which it is currently
formatted to. PI Types cannot coexist on a single drive. A drive can only be formatted to a single PI Type. It can
be changed at anytime to a new Type but requires a low level format which destroys all existing data on the
drive. No other vehicle for changing the PI type is provided by the T10 SBC3 specification.
Type 1 PI format CDB command: 04 90 00 00 00 00, Write Buffer: 00 A0 00 00
Type 2 PI format CDB command: 04 D0 00 00 00 00, Write Buffer: 00 A0 00 00