Re: sg regression in 2.6.16-rc5

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On Feb 28, 2006, at 2:38 PM, Kai Makisara wrote:

On Tue, 28 Feb 2006, Douglas Gilbert wrote:

Mark,
You can stop right there with the 1 MB reads. Welcome
to the new, blander sg driver which now shares many
size shortcomings with the block subsystem.

In lk 2.6.15 the sg driver (and the st driver) did its
own scatter gather list allocations. The sg driver
used 32 KB segments (8 times the normal page size)
in each scatter gather element. The maximum number
of scatter gather elements depends on the LLD but
can be no more than 256. That meant the sg driver
allowed a maximum single IO size of 8 MB. There was
also a define in sg.h (SG_SCATTER_SZ and it is still
there) that allowed the 32KB per segment to be increased
allowing larger single command transfers (then 8 MB).

This is still possible but it needs some changes to most SCSI HBA drivers. The big requests are split into bios supporting 256 pages. For 4 kB pages, this limits i/o to 1 MB. The scsi_execute_async() path used by st and sg can chain bios and this enables large request at the ULD level. At lower level, the request consists of pages and now we hit the s/g list maximum
length _unless_ the HBA driver enables clustering. In this case the
adjacent pages are coalesced and the large requests fit into the HBA s/g limits. Well, now we hit another limit: the max_sectors default for SCSI drivers is 1024 and this limits requests to 512 kB _unless_ the HBA driver
increases max_sectors.

The aic79xx driver enables clustering but does not increase max_sectors.
This makes the maximum request size 512 kB. If it is possible to set

	.max_sectors = 0xFFFF,

in linux/drivers/scsi/aic7xxx/aic79xx_osm.c without breaking the driver, this should enable requests up to 8 MB - 256 B. (I don't have the hardware
to test this.)

Indeed, this seems to work fine, at least with the hardware we have. Gotta love those one-line patches.

Several SCSI HBA drivers currently have similar problems.

Yes. Now that I know about this, it is no problem. I'm not allergic to patches.

Thanks to both of you for your responses. I would submit a patch for this except that I know I don't know that it won't cause problems for configurations and targets that I can't test.

--
Mark Rustad, [email protected]

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