i386/74074: hw.ata.wc=0 / but write cache still enabled

[Arne Wörner]

>Number:         74074
>Category:       i386
>Synopsis:       hw.ata.wc=0 / but write cache still enabled
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-i386
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Thu Nov 18 14:10:27 GMT 2004
>Closed-Date:
>Last-Modified:
>Originator:     Arne Wörner
>Release:        R5.3
>Organization:
>Environment:
FreeBSD neo.riddick.homeunix.org. 5.3-RELEASE FreeBSD 5.3-RELEASE #0: Thu Nov 11 17:27:53 UTC 2004     aw at neo.riddick.homeunix.org.:/usr/obj/usr/src/sys/RIDDICK  i386
>Description:
I wanted to turn of ATA disc write caching (by setting hw.ata.wc to zero):
but after a boot after power on atacontrol cap 0 0 still says, that the write cache feature is enabled:
       write cache                    yes      yes
while sysctl says, that hw.ata.wc is 0:
       # sysctl hw.ata.wc
       hw.ata.wc: 0

I think this is a bug, that can be security relevant in the case of a power failure or system crash (at least if one is using a filesystem with soft updates enabled).

After a reboot (warm) the write cache is sometimes disabled but sometimes enabled... :-(

>How-To-Repeat:
1. shutdown the box
2. turn off power
3. turn on power
4. look at hw.ata.wc
5. look ata atacontrol's output
>Fix:
      
>Release-Note:
>Audit-Trail:
>Unformatted: