Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Matthew Wilcox <matthew@wil.cx>]

On Thu, Dec 13, 2007 at 01:37:59PM -0500, Mark Lord wrote:
> The problem is, the block layer *never* sends an SG entry larger than 8192 
> bytes,
> and even that size is exceptionally rare.  Nearly all I/O segments are 4096 
> bytes,
> so I never see a single I/O larger than 512KB (128 * 4096).
> 
> If I patch various parts of block and SCSI, this limit doesn't budge,
> but when I change the hardware PRD limit in libata, it scales by exactly
> whatever I set the new value to.  This tells me that adjacent I/O segments
> are not being combined.
> 
> I thought that QUEUE_FLAG_CLUSTER (aka. SCSI host .use_clustering=1) should
> result in adjacent single pages being combined into larger physical 
> segments?

I was recently debugging a driver and noticed that consecutive pages in
an sg list are in the reverse order.  ie first you get page 918, then
917, 916, 915, 914, etc.  I vaguely remember James having patches to
correct this, but maybe they weren't merged?

-- 
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"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
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