Subject: Re: Adding B_WRITETHRU -- file system question
To: Manuel Bouyer <>
From: Alfred Perlstein <>
List: tech-kern
Date: 12/04/2001 13:55:50
* Manuel Bouyer <> [011204 13:50] wrote:
> On Mon, Dec 03, 2001 at 02:23:13PM -0800, Jason R Thorpe wrote:
> > 
> > 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.
> Hum, I'm not sure how to enforce this for IDE. I guess we should just disable
> the write cache for IDE disks.


we tried that under FreeBSD, the 4.3 release was so ungodly slow
on IDE disks because of it that we got a LOT of negative PR.

Linux keeps it on, FreeBSD keeps it on, I think you guys should
consider leaving it on just to avoid the pain for the users.

Although it's less correct to keep write caching on, it's probably
better, a consumer purchasing IDE equipment usually is going
commodity and want's the best bang for his/her buck no matter
the consequences.

<possible FUD>
One last point, I seem to remeber our IDE guru lamenting the fact
that even though he turned writecache off, some "smart" drives
would in fact turn it back on if there was extremely heavy IO
pressure on the drive.
</possible FUD>

-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.'