There seems to be some sort of race condition between blkdev_issue_zeroout() and the scsi disk driver (disabling write same after an illegal request). On my UAS drive, sometimes `blkdiscard -z /dev/sdX` will return right away, even though if I then check `write_same_max_bytes` it has turned 0. Sometimes it will just write zero with SCSI WRITE even if `write_same_max_bytes` is 33553920 before I issue `blkdiscard -z` (`write_same_max_bytes` also turned 0, as expected). Not sure if it is directly related to the case here though. On 27 May 2016 at 12:45, Sitsofe Wheeler <sitsofe@xxxxxxxxx> wrote: > On 27 May 2016 at 05:18, Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote: >> >> It's possible that the pvscsi device advertised WRITE SAME, but if the device >> sends back ILLEGAL REQUEST then the SCSI disk driver will set >> write_same_max_bytes=0. Subsequent BLKZEROOUT attempts will then issue writes >> of zeroes to the drive. > > Thanks for following up on this but that's not what happens on the md > device - you can go on to issue as many BLKZEROOUT requests as you > like but the md disk is never zeroed nor is an error returned. > > I filed a bug at https://bugzilla.kernel.org/show_bug.cgi?id=118581 > (see https://bugzilla.kernel.org/show_bug.cgi?id=118581#c6 for > alternative reproduction steps that use scsi_debug and can be reworked > to impact device mapper) and Shaohua Li noted that > blkdev_issue_write_same could return 0 even when the disk didn't > support write same (see > https://bugzilla.kernel.org/show_bug.cgi?id=118581#c8 ). > > Shaohua went on to create a patch for this ("block: correctly fallback > for zeroout" - https://patchwork.kernel.org/patch/9137311/ ) which has > yet to be reviewed. > > -- > Sitsofe | http://sucs.org/~sits/ > -- > 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 -- 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
