hesela.devSCSI COMPARE AND WRITE
SCSI COMPARE AND WRITE conditionally replaces a logical-block range when its contents match supplied comparison data, performing comparison and conditional replacement as one uninterrupted operation.
The host supplies expected and replacement buffers for the same LBA range; a mismatch reports MISCOMPARE without performing the replacement. The device executes comparison and conditional writing without interruption, unlike separate host read and write commands. [1]
The Block Limits VPD page reports MAXIMUM COMPARE AND WRITE LENGTH in logical blocks. A zero value in this field means the command is unsupported; a nonzero value is the largest accepted block count, not a byte count. [2]
The documented sg_compare_and_write utility calculates its default data-out length assuming 512 bytes per logical block, without querying READ CAPACITY. Other block sizes and protection-information settings require appropriate transfer sizing. [1]
In the pinned sg3_utils implementation, MISCOMPARE with valid sense Information reports the first differing byte offset; without valid Information it reports a mismatch without an offset. This byte offset is not a failing LBA. [3]
Hesela's interpretation: this conditional block update does not by itself make a multi-command workflow transactional or establish its power-loss durability; those require separate persistence and recovery contracts.
Relations
- related
- Compare-and-swap, NVMe Compare and Write, Conditional write, Force Unit Access, Logical block address, SCSI Information field
Citations
- sg3_utils: sg_compare_and_write manual, DESCRIPTION and OPTIONS; source commit 8799caa3741b2b1640970747aeb84f17dbbaad86.
- Seagate SCSI Commands Reference Manual, 100293068 Rev. L, November 2020, section 5.4.5, Table 520 and page 534.
- sg3_utils: sg_compare_and_write.c, scsi_compare_and_write MISCOMPARE branch; commit 8799caa3741b2b1640970747aeb84f17dbbaad86.
Sources & caveats
- Source-based command explanation, not a device conformance or power-failure test.
- The sg3_utils source is commit-pinned; its manpage header is not treated as the current package version.
- Seagate Rev. L is used for the Block Limits VPD field, not as evidence that every SCSI device implements the optional command.