[Qemu-devel] write_zeroes/trim on the whole disk

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Hi all!

There is a following problem. When we need to write_zeroes or trim the 
whole disk, we have to do it iteratively, because of 32-bit restriction 
on request length.
For example, current implementation of mirror (see mirror_dirty_init()) 
do this by chunks of 2147418112 bytes (with default granularity of 
65536). So, to zero 16tb disk we will make 8192 requests instead of one.

Incremental zeroing of 1tb qcow2 takes > 80 seconds for me (see below). 
This means ~20 minutes for copying empty 16tb qcow2 disk which is 
obviously a waste of time.

We see the following solutions for nbd:
||
1. Add command NBD_MAKE_EMPTY, with flag, saying what should be done: 
trim or write_zeroes.
2. Add flag NBD_CMD_FLAG_WHOLE for commands NBD_TRIM and 
NBD_WRITE_ZEROES, which will say (with zeroed offset and lenght of the 
request), that the whole disk should be discarded/zeroed.
3. Increase length field of the request to 64bit.

As soon as we have some way to empty disk  in nbd, we can use 
qcow2_make_empty, to trim the whole disk (and something similar should 
be done for zeroing).

What do you think about this all, and which way has a chance to get into 
nbd proto?


== test incremental qcow2 zeroing in mirror ==

1. enable it. If we will use nbd, it will be enabled.
diff --git a/block/mirror.c b/block/mirror.c
index f9d1fec..4ac0c39 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -556,7 +556,7 @@ static int coroutine_fn 
mirror_dirty_init(MirrorBlockJob *s)

      end = s->bdev_length / BDRV_SECTOR_SIZE;

-    if (base == NULL && !bdrv_has_zero_init(target_bs)) {
+    if (base == NULL) {
          if (!bdrv_can_write_zeroes_with_unmap(target_bs)) {
              bdrv_set_dirty_bitmap(s->dirty_bitmap, 0, end);
              return 0;


==== test ====
qemu-img create  -f qcow2 /tmp/1tb.qcow2 1T

virsh start backup-vm --paused
Domain backup-vm started

virsh qemu-monitor-command backup-vm 
{"execute":"blockdev-add","arguments":{"options": {"aio": "native", 
"file": {"driver": "file", "filename": "/tmp/1tb.qcow2"}, "discard": 
"unmap", "cache": {"direct": true}, "driver": "qcow2", "id": "disk"}}}
{"return":{},"id":"libvirt-32"}

/usr/bin/time -f '%e seconds' sh -c 'virsh qemu-monitor-event' &
virsh qemu-monitor-command backup-vm 
{"execute":"drive-mirror","arguments":{"device": "disk", "sync": "full", 
"target": "/tmp/targ"}}
{"return":{},"id":"libvirt-33"}

[root@kvm qemu]# event BLOCK_JOB_READY at 1474652677.668624 for domain 
backup-vm: 
{"device":"disk","len":1099511627776,"offset":1099511627776,"speed":0,"type":"mirror"}
events received: 1

86.39 seconds

- the same for 2tb empty disk: 180.19 seconds
- and without patch, it takes < 1 second, of course.

-- 
Best regards,
Vladimir