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. -- 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