Date: Sun, 5 May 1996 03:55:53 +0300 (EET DST) From: Heikki Suonsivu <hsu@clinet.fi> To: hdalog@zipnet.net Cc: freebsd-scsi@freebsd.org Subject: Re: Buffered Writes? Message-ID: <199605050055.DAA13322@cantina.clinet.fi> In-Reply-To: Peter Dufault's message of 5 May 1996 00:52:25 %2B0300 References: <199605042133.RAA20783@hda.com>
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From: Peter Dufault <dufault@hda.com>
> Does anybody know how to or have a utility to turn on the
> write cache on a barracuda? I have a wide barracuda on a NCR 825 and
You would use the "-e" option of the mode page editor to edit mode
page 8 and set the "WCE" (write cache enable) field to 1 to enable
write back cacheing. Do "man 8 scsi".
This will be similar to the async update of your file system meta
data - you will introduce the problem of deferred errors, which
presently will give you an error on the wrong disk transfer and
treat the failed transfer as successful, and you'll have the
possibility of not writing out the buffer on the disk in the case
of power failure (unless your drive can write during spindown).
queue algorithm modifier usually is "write in issue order" so just setting
WCE to 1 does not do cause any damage (and nor it usually improves the
performance if the load is continous like news disk). But the more
important issue is that I still haven't seen a disk which actually *does*
have a write cache (pointers welcome :). Easy test is to write data to
every second block on the disk. If there is a write cache, you should get
more than block / ~70ms (one disk revolution) written, optimally you should
get the disk platter speed / 2, but scsi handshake overhead usually makes
this impossible.
--
Heikki Suonsivu, T{ysikuu 10 C 83/02210 Espoo/FINLAND, hsu@clinet.fi
mobile +358-40-5519679 work +358-0-4375360 fax -4555276 home -8031121
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