Re: sg regression in 2.6.16-rc5

[Douglas Gilbert <dougg@torque.net>]

Linus Torvalds wrote:
> 
> On Wed, 1 Mar 2006, Matthias Andree wrote:
> 
>>On Tue, 28 Feb 2006, Douglas Gilbert wrote:
>>
>>
>>>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.
>>
>>What is the reason to break user-space applications like this?
> 
> 
> Did you read the whole thread? It was a low-level SCSI driver issue, where 
> nothing broke user space, but the command was just fed to the drive 
> differently, which then hit a limit in the driver.

Linus,
That is an optimistic take. The maximum data carrying
capacity of a single SCSI command via the SG_IO ioctl
depends on the maximum data carrying capacity of a
scatter gather list. Assuming all scatter gather list
elements carry the same amount of data then the
maximum capacity is:
'max_bytes_per_element * max_num_elements'

Only the latter figure is a "low-level SCSI driver issue"
whose maximum seems to be SG_ALL (255). It is the former
figure that has changed. The sg driver in lk 2.6.15 used
__get_free_pages() with the order set to get 32 KB where
as the generic routine used now get a single page (usually
4 KB). Kai Makisara proposed changes in the SCSI LLD
template that made things better in my experiments with
scsi_debug.

However today James Bottomley confirmed that relying on
coalescing pages that may be adjacent is not deterministic:
http://marc.theaimsgroup.com/?l=linux-scsi&m=114122991606658&w=2

That leaves a worst case scatter gather list data capacity
of (4 * 255) KB if the SCSI LLD (or SATA) uses SG_ALL. That
is still just under the 1 MB bar that started this thread.

So I guess we might find out how many people do big,
single SCSI command data, transfers when lk 2.6.16 comes
out.

Doug Gilbert
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