On Tue, 23 Feb 2010, Martin K. Petersen wrote: > >>>>> "Alan" == Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> writes: > > Alan> James or Jens, is there a reason why this routine limits > Alan> max_sectors to 1024? > > Alan> And is there a reason why it sets both max_sectors and > Alan> max_hw_sectors, leaving no way to change one without the other? > > The rationale being that we don't want the default value to be too large > because it makes I/O choppy on single-spindle setups. > > If the device driver supports bigger requests we set the hard limit to > whatever they support. But filesystem I/O is capped at the default > limit which is 1024 for modern devices (and 255 for crappy ones). > > You can change the current soft limit on a per-device basis in > /sys/block/<dev>/queue/max_sectors_kb. > > I agree it would be nice to have a system-wide default that's not a > #define. But given that a lot of queues are created early that's not so > easy to do. > > > Alan> Are drivers supposed to assign values directly to > Alan> q-> limits.max_sectors? Shouldn't there be a cleaner API for all > Alan> this? > > Drivers are supposed to set the hard limit. The soft limit is a > fs/block layer value and none of the device driver's business. So Ramya, it looks like you need to edit store_max_sectors() in drivers/usb/storage/scsiglue.c. Get rid of the " && ms <= SCSI_DEFAULT_MAX_SECTORS" part. Then you should be able to write a large value to /sys/block/sdX/device/max_sectors to increase the hardware limit. The new value divided by 2 will show up in /sys/block/sdX/queue/max_hw_sectors_kb. Once that limit is increased, you can write a large value to /sys/block/sdX/queue/max_sectors_kb. That should give you long transfer lengths. Alan Stern -- To unsubscribe from this list: 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
