Mark Rustad wrote: > We have encountered some kind of sg regression with kernel 2.6.16-rc5 > relative to 2.6.15. We have a small program that demonstrates the > failure. On 2.6.15 it produces the output: > > Alloced dataptr 0 -> 0xb7d07008 > IOS: 0 > ios 100 > > indicating that it did 100 operations successfully. On 2.6.16-rc5, it > produces the output: > > Alloced dataptr 0 -> 0xa7d10008 > SG_IO ioctl error 12 Cannot allocate memory > ios 0 > > indicating that it did 0 operations successfully. This program is > attempting to do 1MB reads on a SCSI device. 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). We get the failure both on > an aic79xx parallel SCSI and on aic94xx SAS. With both types of > devices, it works fine on the 2.6.15 kernel. We have also seen this > problem on the 2.6.16-rc4 kernel. In all cases we were running on an > Intel Xeon-based system. Well this is broken by design. If you and others talk to the management it may be reversed or a better solution may be found. Here is an example of the sg driver in lk 2.6.15-rc5. The number of bytes each SCSI READ command is trying to fetch is 'bs * bpt'. Note that it works for 256 KB per SCSI READ but fails for anything bigger: # modprobe scsi_debug # modprobe sg # sg_dd if=/dev/sg0 of=. bs=512 bpt=512 16384+0 records in 16384+0 records out # sg_dd if=/dev/sg0 of=. bs=512 bpt=513 sg_read failed, try reducing bpt, at or after lba=0 [0x0] Some error occurred, remaining block count=16384 0+0 records in 0+0 records out # sg_dd if=/dev/sg0 of=. bs=512 bpt=1024 sg_read failed, try reducing bpt, at or after lba=0 [0x0] Some error occurred, remaining block count=16384 0+0 records in 0+0 records 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
