QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

[Mark Lord <lkml@rtr.ca>]

Jens,

I'm experimenting here with trying to generate large I/O through libata,
and not having much luck.

The limit seems to be the number of hardware PRD (SG) entries permitted
by the driver (libata:ata_piix), which is 128 by default.

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?

This is x86-32 with latest 2.6.24-rc*.
I'll re-test on older kernels next.

???
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