hesela.devATA read look-ahead
ATA read look-ahead is a drive-controlled read-prefetch feature, separate from operating-system readahead and the drive's write-cache setting.
The hdparm controls differ by case: -a selects a software filesystem readahead sector count, whereas -A selects the drive feature. Changing one does not configure the other layer. [1]
SET FEATURES uses command opcode 0xEF with feature selector 0xAA to enable read look-ahead or 0x55 to disable it. Write-cache selectors 0x02 and 0x82 are separate; a read-prefetch change is not a write-cache flush. [2] [4]
IDENTIFY word 82 bit 6 reports support; word 85 bit 6 reports enablement. Linux v6.12 checks the validity pattern in word 87 before interpreting that enable bit. Neither field measures fetched bytes, cache hits or application latency. [3] [2]
The cited product manual says these settings revert to defaults after power cycling. That is a documented product behavior, not a guarantee about every ATA device or reset type. [4]
Relations
Limitations
- A host command path may not expose every device setting; enclosure and kernel compatibility matter. [1]
Citations
- hdparm(8): separate -a software readahead and -A drive read-lookahead controls
- Linux v6.12 ata.h: SET FEATURES selectors and ata_id_rahead_enabled validity check
- Linux v6.12 hdreg.h: IDENTIFY words 82 and 85, support versus enabled state
- WD Purple Pro PR18 manual D018-000157-AA00: Tables 4-6 and 4-7, pages 36-37 and 41
Further reading
Sources & caveats
- Documentation and version-pinned Linux source review; no device command, cache-hit measurement or performance experiment was performed.
- The power-cycle statement is limited to the WD Purple Pro PR18 manual. No universal default or reset-persistence behavior is inferred.