On Thu, Feb 16 2012 at 3:02pm -0500, Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > FYI, I'll bounce a message detailing the iSCSI scatter-gather NULL > pointer I _always_ hit with dm-io issuing async WRITE_SAME. I developed a patch for dm-io so that the new dm-thinp target can leverage your new WRITE SAME functionality for, hopefully, more efficient zeroing of the disk (see: dm-io-WRITE_SAME.patch at the end of the following patchset). Here is the patchset I'm using ontop of Linux 3.2: http://people.redhat.com/msnitzer/patches/upstream/dm-io-WRITE_SAME/series.html All works great on FC (tested against NetApp 3040 LUN)... I'm using the thinp-test-suite to test dm-thinp's use of dm_kcopyd_zero(). But testing with iSCSI, I get a NULL pointer _every_ time in the iSCSI scatter-gather code, see: http://people.redhat.com/msnitzer/patches/upstream/dm-io-WRITE_SAME/async-WRITE_SAME-makes-iscsi-sg-die.txt -- in the middle of that file you'll see my 'crash' analysis of the issue -- but that is just the NULL pointer.. no idea what the smoking gun is that caused the iscsi_segment to become NULL. Anyway, taking a step back... WRITE SAME is all about transfering a single logical block, backed by a single empty_zero_page in this test case, so I'm wondering if for some reason iSCSI's sg code is getting confused and thinking that more pages need to be transferred than were in the original bio's payload (but iSCSI is way beneath the bio -> SCSI command translation... grr) It should be noted that using your synchronous blkdev_issue_write_same() interface works fine on the same iSCSI LUN (again NetApp 3040 LUN): http://people.redhat.com/msnitzer/patches/upstream/dm-io-WRITE_SAME/dm-thin-use-WRITE_SAME-for-zeroing.patch (but an dm-thinp really wants an async interface for zeroing). Mike p.s. dm-io-WRITE_SAME.patch's use of dp->get_page()+bio_add_page() accomplishes the same bio init as you do in blkdev_issue_write_same(): bio->bi_vcnt = 1; bio->bi_phys_segments = 1; bio->bi_io_vec->bv_page = virt_to_page(empty_zero_page); bio->bi_io_vec->bv_offset = offset_in_page(empty_zero_page); bio->bi_io_vec->bv_len = logical_block_size; -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
