Re: [PATCH] 2.4.x write barriers (updated for ext3)

[Dieter Nützel]

James Bottomley wrote:
> [e-mail] said:
> > So, a little testing with scsi_info shows my scsi drives do have
> > writeback cache on.  great.  What's interesting is they must be doing
> > additional work for ordered tags.  If they were treating the block as
> > written once in cache, using the tags should not change  performance
> > at all.  But, I can clearly show the tags changing performance, and
> > hear the drive write pattern change when tags are on. 

> I checked all mine and they're write through.  However, I inherited all my 
> drives from an enterprise vendor so this might not be that surprising.

How do you checked it?
Which scsi_info version?
Mine gave only the below info:

SunWave1 /home/nuetzel# scsi_info /dev/sda
SCSI_ID="0,0,0"
MODEL="IBM DDYS-T18350N"
FW_REV="S96H"
SunWave1 /home/nuetzel# scsi_info /dev/sdb
SCSI_ID="0,1,0"
MODEL="IBM DDRS-34560D"
FW_REV="DC1B"
SunWave1 /home/nuetzel# scsi_info /dev/sdc
SCSI_ID="0,2,0"
MODEL="IBM DDRS-34560W"
FW_REV="S71D"

But when I use "scsi-config" I get under "Cache Control Page":
Read cache enabled: Yes
Write cache enabled: No

I've tested it with setting this by hand some months ago, but the speed 
doesn't change in anyway (ReiserFS).

> I can surmise why ordered tags kill performance on your drive, since an 
> ordered tag is required to affect the ordering of the write to the medium,
> not the cache, it is probably implemented with an implicit cache flush.
>
> Anyway, the attached patch against 2.4.18 (and I know it's rather gross
> code)  will probe the cache type and try to set it to write through on boot.
>  See what this does to your performance ordinarily, and also to your
> tagged write barrier performance.

Will test it over the weekend on 2.4.19-pre1aa1 with all Reiserfs 
2.4.18.pending patches applied.

Regards,
	Dieter