Re: [Qemu-block] [Qemu-devel] [PATCH for-2.12 0/4] qmp dirty bitmap API

[Vladimir Sementsov-Ogievskiy]

26.12.2017 10:07, Fam Zheng wrote:
> On Wed, 12/20 11:20, Vladimir Sementsov-Ogievskiy wrote:
> > external backup:
> >
> > 0. we have active_disk and attached to it dirty bitmap bitmap0
> > 1. qmp blockdev-add tmp_disk (backing=active_disk)
> > 2. guest fsfreeze
> > 3. qmp transaction:
> >          - block-dirty-bitmap-add node=active_disk name=bitmap1
> >          - block-dirty-bitmap-disable node=active_disk name=bitmap0
> >          - blockdev-backup drive=active_disk target=tmp_disk sync=none
> > 4. guest fsthaw
> > 5. (? not designed yet) qmp blockdev-add filter_node - special filter node
> > over tmp_disk for synchronization of nbd-reads and backup(sync=none) cow
> > requests (like it is done in block/replication)
> > 6. qmp nbd-server-start
> > 7. qmp nbd-server-add filter_node (there should be possibility of exporting
> > bitmap of child node filter_node->tmp_disk->active_disk->bitmap0)
> >
> > then, external tool can connect to nbd server and get exported bitmap and
> > read data (including bitmap0) accordingly to nbd specification.
> > (also, external tool may get a merge of several bitmaps, if we already have
> > a sequence of them)
> > then, after backup finish, what can be done:
> >
> > 1. qmp block-job-cancel device=active_disk (stop our backup(sync=none))
> > 2. qmp nbd-server-stop (or qmp nbd-server-remove filter_node)
> > 3. qmp blockdev-remove filter_node
> > 4. qmp blockdev-remove tmp_disk
> >
> > on successful backup, you can drop old bitmap if you want (or do not drop
> > it, if you need to keep sequence of disabled bitmaps):
> > 1. block-dirty-bitmap-remove node=active_disk name=bitmap0
> >
> > on failed backup, you can merge bitmaps, to make it look like nothing
> > happened:
> > 1. qmp transaction:
> >         - block-dirty-bitmap-merge node=active_disk name-source=bitmap1
> > name-target=bitmap0
> Being done in a transaction, will merging a large-ish bitmap synchronously hurt
> the responsiveness? Because we have the BQL lock held here which pauses all
> device emulation.
>
> Have you measured how long it takes to merge two typical bitmaps. Say, for a 1TB
> disk?
>
> Fam

We don't need merge in a transaction.

Anyway, good question.

two full of ones bitmaps, 64k granularity, 1tb disk:
# time virsh qemu-monitor-command tmp '{"execute": 
"block-dirty-bitmap-merge", "arguments": {"node": "disk", "src_name": 
"a", "dst_name": "b"}}'
{"return":{},"id":"libvirt-1181"}
real    0m0.009s
user    0m0.006s
sys     0m0.002s

and this is fine:
for last level of hbitmap we will have
   disk_size / granularity / nb_bits_in_long = (1024 ^ 4) / (64 * 1024) 
/ 64 = 262144
oparations, which is quite a few



bitmaps in gdb:

(gdb) p bdrv_lookup_bs ("disk", "disk", 0)
$1 = (BlockDriverState *) 0x7fd3f6274940
(gdb) p *$1->dirty_bitmaps.lh_first
$2 = {mutex = 0x7fd3f6277b28, bitmap = 0x7fd3f5a5adc0, meta = 0x0, 
successor = 0x0,
  name = 0x7fd3f637b410 "b", size = 1099511627776, disabled = false, 
active_iterators = 0,
  readonly = false, autoload = false, persistent = false, list = 
{le_next = 0x7fd3f567c650,
    le_prev = 0x7fd3f6277b58}}
(gdb) p *$1->dirty_bitmaps.lh_first ->bitmap
$3 = {size = 16777216, count = 16777216, granularity = 16, meta = 0x0, 
levels = {0x7fd3f6279a90,
    0x7fd3f5506350, 0x7fd3f5affcb0, 0x7fd3f547a860, 0x7fd3f637b200, 
0x7fd3f67ff5c0, 0x7fd3d8dfe010},
  sizes = {1, 1, 1, 1, 64, 4096, 262144}}
(gdb) p *$1->dirty_bitmaps.lh_first ->list .le_next
$4 = {mutex = 0x7fd3f6277b28, bitmap = 0x7fd3f567cb30, meta = 0x0, 
successor = 0x0,
  name = 0x7fd3f5482fb0 "a", size = 1099511627776, disabled = false, 
active_iterators = 0,
  readonly = false, autoload = false, persistent = false, list = 
{le_next = 0x0,
    le_prev = 0x7fd3f6c779e0}}
(gdb) p *$1->dirty_bitmaps.lh_first ->list .le_next ->bitmap
$5 = {size = 16777216, count = 16777216, granularity = 16, meta = 0x0, 
levels = {0x7fd3f5ef8880,
    0x7fd3f5facea0, 0x7fd3f5f1cec0, 0x7fd3f5f40a00, 0x7fd3f6c80a00, 
0x7fd3f66e5f60, 0x7fd3d8fff010},
  sizes = {1, 1, 1, 1, 64, 4096, 262144}}

--
Best regards,
Vladimir