Subject: Re: Adding B_WRITETHRU -- file system question
To: Jason R Thorpe <>
From: Alfred Perlstein <>
List: tech-kern
Date: 12/03/2001 18:10:49
* Jason R Thorpe <> [011203 16:23] wrote:
> Warping back to a conversation I had here some time ago...
> I want to add a flag to struct buf which indicates that an I/O
> transaction is to be done in a "cache write-through" manner.
> This is pretty easy to accomplish, for SCSI at least... Set the
> Force Unit Access bit when you see B_WRITETHRU, and then clear
> B_WRITETHRU when the transfer is finished.
> Comments?

I wanted to do this in FreeBSD a while back but couldn't get concensus
from the device people that the flag would be honored.  There was
also the issue that a non-B_WRITETHRU block for the same address
may be sorted either before or after the B_WRITETHRU block possibly
blindsiding code that relies upon it working correctly.

-Alfred Perlstein []
'Instead of asking why a piece of software is using "1970s technology,"
 start asking why software is ignoring 30 years of accumulated wisdom.'