This is because of the min_size specification. I would bet you have it set at 2 (which is good).
ceph osd pool get rbd min_size
With 4 hosts, and a size of 3, removing 2 of the hosts (or 2 drives 1 from each hosts) results in some of the objects only having 1 replica
min_size dictates that IO freezes for those objects until min_size is achieved. http://docs.ceph.com/docs/jewel/rados/operations/pools/#set-the-number-of-object-replicas
I cant tell if your under the impression that your RBD device is a single object. It is not. It is chunked up into many objects and spread throughout the cluster, as Kjeti mentioned earlier.
On Mon, Mar 20, 2017 at 8:48 PM, Kjetil Jørgensen <kjetil@xxxxxxxxxxxx> wrote:
Hi,rbd_id.vm-100-disk-1 is only a "meta object", IIRC, it's contents will get you a "prefix", which then gets you on to rbd_header.<prefix>, rbd_header.prefix contains block size, striping, etc. The actual data bearing objects will be named something like rbd_data.prefix.%-016x.Example - vm-100-disk-1 has the prefix 86ce2ae8944a, the first <block size> of that image will be named rbd_data. 86ce2ae8944a.000000000000, the second <block size> will be 86ce2ae8944a.000000000001, and so on, chances are that one of these objects are mapped to a pg which has both host3 and host4 among it's replicas.An rbd image will end up scattered across most/all osds of the pool it's in.Cheers,-KJ--On Fri, Mar 17, 2017 at 12:30 PM, Adam Carheden <carheden@xxxxxxxx> wrote:I have a 4 node cluster shown by `ceph osd tree` below. Monitors are
running on hosts 1, 2 and 3. It has a single replicated pool of size
3. I have a VM with its hard drive replicated to OSDs 11(host3),
5(host1) and 3(host2).
I can 'fail' any one host by disabling the SAN network interface and
the VM keeps running with a simple slowdown in I/O performance just as
expected. However, if 'fail' both nodes 3 and 4, I/O hangs on the VM.
(i.e. `df` never completes, etc.) The monitors on hosts 1 and 2 still
have quorum, so that shouldn't be an issue. The placement group still
has 2 of its 3 replicas online.
Why does I/O hang even though host4 isn't running a monitor and
doesn't have anything to do with my VM's hard drive.
Size?
# ceph osd pool get rbd size
size: 3
Where's rbd_id.vm-100-disk-1?
# ceph osd getmap -o /tmp/map && osdmaptool --pool 0 --test-map-object
rbd_id.vm-100-disk-1 /tmp/map
got osdmap epoch 1043
osdmaptool: osdmap file '/tmp/map'
object 'rbd_id.vm-100-disk-1' -> 0.1ea -> [11,5,3]
# ceph osd tree
ID WEIGHT TYPE NAME UP/DOWN REWEIGHT PRIMARY-AFFINITY
-1 8.06160 root default
-7 5.50308 room A
-3 1.88754 host host1
4 0.40369 osd.4 up 1.00000 1.00000
5 0.40369 osd.5 up 1.00000 1.00000
6 0.54008 osd.6 up 1.00000 1.00000
7 0.54008 osd.7 up 1.00000 1.00000
-2 3.61554 host host2
0 0.90388 osd.0 up 1.00000 1.00000
1 0.90388 osd.1 up 1.00000 1.00000
2 0.90388 osd.2 up 1.00000 1.00000
3 0.90388 osd.3 up 1.00000 1.00000
-6 2.55852 room B
-4 1.75114 host host3
8 0.40369 osd.8 up 1.00000 1.00000
9 0.40369 osd.9 up 1.00000 1.00000
10 0.40369 osd.10 up 1.00000 1.00000
11 0.54008 osd.11 up 1.00000 1.00000
-5 0.80737 host host4
12 0.40369 osd.12 up 1.00000 1.00000
13 0.40369 osd.13 up 1.00000 1.00000
--
Adam Carheden
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Respectfully,
Wes Dillingham
Research Computing | Infrastructure Engineer
Harvard University | 38 Oxford Street, Cambridge, Ma 02138 | Room 210
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