On Sun, 1 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... Almost true. :) The driver will not fire any ordered tag if all IOs complete within 3 seconds (3 seconds delay hard-coded). In fact the driver uses 2 counters of tagged commands. When switching to the other counter (mostly every 3 seconds), if this counter is not zero then the value represents the number of tagged commands that haven't completed within at least 3 seconds (and at most something lower than 6 seconds). The message may well be harmless. But since it is printed out with verbosity=1, the syslog might be flooded a bit. User can hack the driver for this message to be printed out for higher verbosity, or ensure verbosity is minimal (Verbose mode not set in NVRAM and boot with sym53c8xx=verb:0). > 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. Indeed. My first SCSI disk was an IBM S12 that starved tagged commands a lot. But I have seen recent hard disks reordering IOs heavily too (hundreds of IOs being passed). But they are so fast than the 3 seconds delay is unlikely to be hit very often. This does not apply to raid devices that are slow by design... :-) Regards, Gérard. - To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to [EMAIL PROTECTED]
