docs/52071: [PATCH] Add more information about soft updates into ata(4) manpage

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>Number:         52071
>Category:       docs
>Synopsis:       [PATCH] Add more information about soft updates into ata(4)
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-doc
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          change-request
>Submitter-Id:   current-users
>Arrival-Date:   Sun May 11 05:40:05 PDT 2003
>Closed-Date:
>Last-Modified:
>Originator:     Xin LI <delphij at frontfree.net>
>Release:        FreeBSD 5.1-BETA i386
>Organization:
Frontfree Technology Network
>Environment:
System: FreeBSD beastie.frontfree.net 5.1-BETA FreeBSD 5.1-BETA #53: Wed May 
7 05:03:46 CST 2003 root at beastie.frontfree.net:/usr/obj/usr/src/sys/BEASTIE 
i386

>Description:
	In the ata(4) driver on -CURRENT systems, hw.ata.wc is enabled by default. 
The information supplied with ata(4) only said write cache may cause data 
loss, but it might be insuffient because:
	o With tagged queuing, write cache will not break soft updates.
	o Without tagged queuing, write cache can cause even more data loss if soft 
updates is enabled.
>How-To-Repeat:
>Fix:
	Because the hw.ata.wc is now enabled by default for performance reasons, 
users who are using ata(4) hard drivers should be warned for the fact that 
writing cache may break soft updates because it blinds the soft updates code 
by telling it the data is on stable storage before it really goes to. On 
systems running critical applications, especially databases, this problem 
will be severe.
	In addition, as Dr. McKusick mentioned, on ATA devices, by enabling tagged 
queueing feature, the confilict between write caching and soft updates will 
go away. So document this also.
	This patch also adds two references to: loader(8) and device.hints(5). 
hw.ata.wc and hw.ata.tags are tunnable in loader(8).

	I am not sure if this information should be held in BUGS section, so if I 
was wrong, please change it. Hopefully this patch is useful :P
	Apply the following patch:

--- ata.4.orig	Sun May 11 02:04:11 2003
+++ ata.4	Sun May 11 04:11:16 2003
@@ -245,7 +245,9 @@
older devices do not like this.
.Sh SEE ALSO
.Xr atacontrol 8 ,
-.Xr burncd 8
+.Xr burncd 8 ,
+.Xr loader 8 ,
+.Xr device.hints 5
.Sh HISTORY
The
.Nm
@@ -265,3 +267,21 @@
and
.An S\(/oren Schmidt
.Aq sos at FreeBSD.org .
+.Sh BUGS
+By default the hw.ata.wc is enabled while hw.ata.tags disabled.
+Enabling write cache without tagged queuing is even more risky on
+soft updates enabled file systems, because doing so will cause your
+hardware to tell soft updates code that data was written on disk
+once they are acquired to write cache on drives, rather than when it
+really goes to the stable storage. This is unexpected and tend to
+cause more data loss across system crashes.
+
+However, disabling write cache will dramatically decrease disk
+performance so it is now enabled by default. If your data is *very*
+important, please consider either enable tagged queuing support by
+setting hw.ata.tags to 1, or disable write cache by setting hw.ata.wc
+to 0. Currently, tagged queuing feature is only supported by IBM DPTA,
+DTLA, ICxxxxxxAT, ICxxxxxxAV drives, so you may want to disable
+hw.ata.wc to work around the problem on older hardware. This may be
+accomplished by adding an item in
+.Xr device.hints 5 .

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>Release-Note:
>Audit-Trail:
>Unformatted:
 >manpage