Date: Tue, 20 Feb 2007 09:59:44 -0600 From: Kirk Strauser <kirk@strauser.com> To: freebsd-questions@freebsd.org Subject: Deciphering camcontrol - is prefetch enabled? Message-ID: <200702200959.48154.kirk@strauser.com>
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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 Queue=
ing Enabled
da2: 35003MB (71687372 512 byte sectors: 255H 63S/T 4462C)
=46rom 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.
=2D-=20
Kirk Strauser
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