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 - : send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
