Re: [dm-devel] [PATCH] fix default IO restrictions

[Mike Christie <mikenc us ibm com>]

Christophe Saout wrote:
> Am Mo, den 26.01.2004 schrieb Mike Christie um 22:22:
>
>
> > > I don't think that will happen. The values are already the allowed
> > > maximum values. Or am I wrong?
> >
> > I don't think they are the max values. SCSI will override them depending 
> > on the HBA settings.
>
>
> Hmm. Then I think there is something wrong anyway. Why are the max
> constants not maximum values...

I think...

Why does blkdev.h know the max values? You are saying that they reflect 
a limitation of the kernel?

The queue's max_sectors, max_[phys/hw]_segments, max_segments_size etc 
are device driver/HW limitations. blk_init_queue does not know an 
optimal value for every device, but it might be able to set a safe 
default value. Maybe a better name for MAX_SECTORS is DEFAULT_MAX_SECTORS.

BIO_MAX_[SECTORS/SIZE/PAGES] is what you called a "system-wide limit". A 
bio can only be so large (the bvec_pool is hardcoded). However when 
merging the bio, q->max_sectors is one of the indicators that tell the 
queue when a request is too large for the device.