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Date: Fri, 1 Feb 2013 15:26:58 +0100 (CET)
From: Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>
To: freebsd-hackers@freebsd.org
Subject: HDD write cache
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after reading quite recent topics about disabling/enabling write cache, i 
tried to test in on desktop 3.5" drive

kern.cam.ada.write_cache: 1
kern.cam.ada.read_ahead: 1
kern.cam.ada.0.read_ahead: -1
kern.cam.ada.0.write_cache: -1

i tried writing 1 or 0 to kern.cam.ada.0.write_cache, and there were 
exactly no differences at all, and disk seems to do always write caching.

Does that drive lie and ignore commands or i do it wrong?

this is my disk.

ada0 at ahcich0 bus 0 scbus0 target 0 lun 0
ada0: <SAMSUNG HD501LJ CR100-10> ATA-8 SATA 2.x device
ada0: 300.000MB/s transfers (SATA 2.x, UDMA6, PIO 8192bytes)
ada0: Command Queueing enabled
ada0: 476940MB (976773168 512 byte sectors: 16H 63S/T 16383C)