Deciphering camcontrol - is prefetch enabled?

Kirk Strauser kirk at
Tue Feb 20 15:59:54 UTC 2007

I have a new set of SCSI drives, all like:

    da2 at mpt0 bus 0 target 3 lun 0
    da2: <SEAGATE ST336754LW 0005> Fixed Direct Access SCSI-3 device
    da2: 320.000MB/s transfers (160.000MHz, offset 63, 16bit), Tagged Queueing Enabled
    da2: 35003MB (71687372 512 byte sectors: 255H 63S/T 4462C)

From what I can tell, the only way to adjust parameters on SCSI drives is
through camcontrol's modepage option.  The output on one of the new disks
looks like:

    $ sudo camcontrol modepage 0:2 -m 8
    IC:  0
    ABPF:  0
    CAP:  0
    DISC:  1
    SIZE:  0
    WCE:  1
    MF:  0
    RCD:  0
    Demand Retention Priority:  0
    Write Retention Priority:  0
    Disable Pre-fetch Transfer Length:  65535
    Minimum Pre-fetch:  0
    Maximum Pre-fetch:  65535
    Maximum Pre-fetch Ceiling:  65535

I'm pretty sure that "WCE" means "write cache enabled", which is what we
want.  However, I'm pretty sure I want to enable prefetching, too, since
data access on them will often involve a set of concurrent long streaming
reads.  In that case, wouldn't prefetch reduce contention among the read
processes?  If so, from looking at the above output, does it seem to be
enabled?  Since I left my handy "Cryptic SCSI Acronym Dictionary" elsewhere,
I don't have any idea what the above is actually trying to tell me.
Kirk Strauser
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