On Wed, May 18, 2016 at 11:39:30PM +0100, Sitsofe Wheeler wrote: > Hi, > > With Ubuntu's 4.4.0-22-generic kernel and a Fedora 23 > 4.6.0-1.vanilla.knurd.1.fc23.x86_64 kernel I've found that the > BLKZEROOUT syscall can malfunction and not zero data. > > When BLKZEROOUT is issued to an MD device atop a PVSCSI controller > supplied VMDK from ESXi 6.0 the call returns immediately and with a zero > return code. Unfortunately, inspecting the data on the MD device shows > that it has not been zeroed and is in fact untouched. The easiest way to > see this behaviour is to boot the VM, create an mdadm device atop > /dev/sd?, scribble some non-zero value on the disk and then use > blkdiscard --zeroout /dev/md??? . If you then inspect the MD disk (e.g. > with hexdump) you will still see the old data and using POSIX_FADV_DONTNEED > on the MD device doesn't change the outcome. > > The only clue I've seen is that > /sys/block/sd?/queue/write_same_max_bytes starts out being 33553920 but > after a WRITE SAME is issued it becomes 0. If the MD device is created > after write_same_max_bytes has become 0 on the backing disk then > BLKZEROOUT seems to work correctly. 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. --D > > -- > Sitsofe | http://sucs.org/~sits/ -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
