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SubjectRe: 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, MRustad@mac.com

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