Re: Possible explanation for SCSI benchmark problems in 2.5

[Jens Axboe <axboe@suse.de>]

On Sun, Sep 29 2002, Mike Anderson wrote:
> James Bottomley [James.Bottomley@steeleye.com] wrote:
> > Hi All,
> > 
> > While looking at other code (OK, how to get the MCA drivers not to use bounce 
> > buffers...)  I discovered that the way scsi_scan.c calls 
> > scsi_initialize_merge_fn() guarantees (on an x86) that the 
> > blk_queue_bounce_limit is always called with BLK_BOUNCE_HIGH.  This will 
> > probably hurt benchmarks on machines with 1Gb or more of memory.
> > 
> > The problem is that scsi_initialize_merge_fn() checks sdev->type, but this is 
> > always -1 in scsi_alloc_sdev, and thus it never uses the pci dma mask for disk 
> > devices.
> > 
> > The attached patch moves the initialisation to after sdev->type has the 
> > correct value.
> 
> I thought this was previously discussed on this thread.
> http://marc.theaimsgroup.com/?l=linux-kernel&m=103064539622958&w=2
> 
> and I believe the comment from Jens was that this could be killed
> off in 2.5 if someone did a check for safety on all the devices.

(BTW James, patch looks good)

Well that's one way to interpret it. But it really needs to be a good
audit, I refuse to remove the check if someone just assumes that
sr/st/whatnot are safe. It's simply not worth the risk.

-- 
Jens Axboe