On Tue, 20 Sep 2005, James Bottomley wrote: > On Tue, 2005-09-20 at 22:23 +0300, Kai Makisara wrote: > > > Yeah I think this is due to the MAX_PHYS_SEGMENTS limit. The hw segments is > > > set by scsi_lib in scsi_alloc_queue and is the sg_tablesize value on the host. > > > Right now all I do is return a error when someone violates one of the limits, > > > but I think the right thing to do is have the ULDs take some of these values > > > into account when they build their lists. However if I do that we will not be > > > able to make large requests since the MAX_PHYS_SEGMENTS/SCSI_MAX_PHYS_SEGMENTS > > > will limit them. Umm let me rethink. > > There's already the code in SCSI to increase this to 256 (and > potentially beyond) if it becomes necessary. > The current default segment size limit is 64 kB. This multiplied by 128 gives 8 MB. This should be large enough for all tape usage I know. ... > > What I don't quite like is that being able to do this requires setting > > SCSI adapter parameters (use_clustering, max_sectors) to values that are > > not used by most drivers today. Changing is in most cases trivial but this > > has to be done. Otherwise the users needing large block sizes feel that > > these enhancements are a regression. > > OK, but this is troubling either way: Previously the LLD was simply > lying about its ability to support more sectors and clustering and you > called its bluff by sending such commands. Now the bio layer is > actually enforcing these limits. > I agree that it is good to have a common layer to enforce the limits. Currently st has had to check the limits (it checks some and for others relies on that all decent SCSI HBAs support what st tries to do). Based on my earlier experience, I just doubt a little that the limits will be updated but I hope to be proven wrong here :-) > The fix has got to be to update the LLDs so they're reporting their > capabilities truthfully. > If this happens, it is very good. (I hope this includes the alignment constraints, too :-) -- Kai - : send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
