On Mon, 02 Apr 2001, Ralston, Steve wrote:
> > I get a reasonable number of 'ordered tag forced' messages. As in
> > Apr 2 11:20:34 swtf kernel: sym53c895-0-<3,0>: ordered tag
> forced.
> > This doesn't appear to have any adverse effect on the filesystem
> ...,
> > it's just that I'm curious as to why it is/would happen.
>
> Refer to comment:
> /*
> ** Force ordered tag if necessary to avoid timeouts
> ** and preserve interactivity.
> */
> and following block of code near line 6619 in 2.4.3'ish
> drivers/scsi/sym53c8xx.c.
> Looks like this particular scsi driver throws out ORDERED_TAG every
> 3 seconds or so...
>
> Many scsi drivers inject intermittant ORDERED_TAG like this to keep
> certain SCSI targets from starving WRITE IOs under heavy load.
> Hee, I spent time as "guest" (more like prisoner) of Hitachi in Toyko many
> years
> ago due to a problem of this nature. They were using IBM fast+wide drives
> which had a tendency to defer/starve WRITE IOs for too long under heavy
> load.
>
> Regards,
> -SteveR
Thanks Steve.
Do you know if the inserts of ORDERED_TAG messages(?) are a function of
a. the driver monitoring the proportion of writes verses reads it's processing
and it inserts based on some predetermined formula,
or
b. based on some feedback it DERIVES from the time it takes to process previous
writes.
Regards
Burn
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