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Date:      Fri, 02 Oct 1998 13:46:43 -0600
From:      "Justin T. Gibbs" <gibbs@plutotech.com>
To:        Julian Elischer <julian@whistle.com>
Cc:        Don Lewis <Don.Lewis@tsc.tdk.com>, freebsd-fs@FreeBSD.ORG, freebsd-scsi@FreeBSD.ORG
Subject:   Re: filesystem safety and SCSI disk write caching 
Message-ID:  <199810021953.NAA12817@pluto.plutotech.com>
In-Reply-To: Your message of "Fri, 02 Oct 1998 11:59:26 PDT." <Pine.BSF.3.95.981002115603.15828A-100000@current1.whistle.com> 

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>write caching can screw soft updates if there is any
>major re-ordering of the data written.

Only if you lose power or have a buggy device.  Go read the SCSI spec on
write caching.

>With tags it doesn't matter if they are re-ordered, as long
>as they are not acknowledged until they are on the platter.

Tagged transactions may "complete" in a non-FIFO order. "Complete" either
means data transfered into the cache or data safely on the media depending
on whether the cache is enabled.  Re-ordered writes are allowed, but, only
such that it maintains read/write coherency.  This is with the restrictive
ordering semantics that drives usually ship with by default. You can turn
on "re-order at will" through a mode page.

Waiting for Terry's long winded response to this thread,
Justin



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