For this particular test I turned off replication for both hdfs and ceph.
So there is just one copy of the data lying around.
hadoop@vega7250:~$ ceph osd dump | grep rep
pool 0 'data' rep size 1 min_size 1 crush_ruleset 0 object_hash rjenkins
pg_num 960 pgp_num 960 last_change 26 owner 0 crash_replay_interval 45
pool 1 'metadata' rep size 2 min_size 1 crush_ruleset 1 object_hash
rjenkins pg_num 960 pgp_num 960 last_change 1 owner 0
pool 2 'rbd' rep size 2 min_size 1 crush_ruleset 2 object_hash rjenkins
pg_num 960 pgp_num 960 last_change 1 owner 0
>From hdfs-site.xml:
<property>
<name>dfs.replication</name>
<value>1</value>
</property>
On Tue, Jul 9, 2013 at 2:44 PM, Noah Watkins <noah.watk...@inktank.com>wrote:
> On Tue, Jul 9, 2013 at 12:35 PM, ker can <kerca...@gmail.com> wrote:
> > hi Noah,
> >
> > while we're still on the hadoop topic ... I was also trying out the
> > TestDFSIO tests ceph v/s hadoop. The Read tests on ceph takes about 1.5x
> > the hdfs time. The write tests are worse about ... 2.5x the time on
> hdfs,
> > but I guess we have additional journaling overheads for the writes on
> ceph.
> > But there should be no such overheads for the read ?
>
> Out of the box Hadoop will keep 3 copies, and Ceph 2, so it could be
> the case that reads are slower because there is less opportunity for
> scheduling local reads. You can create a new pool with replication=3
> and test this out (documentation on how to do this is on
> http://ceph.com/docs/wip-hadoop-doc/cephfs/hadoop/).
>
> As for writes, Hadoop will write 2 remote and 1 local blocks, however
> Ceph will write all copies remotely, so there is some overhead for the
> extra remote object write (compared to Hadoop), but i wouldn't have
> expected 2.5x. It might be useful to run dd or something like that on
> Ceph to see if the numbers make sense to rule out Hadoop as the
> bottleneck.
>
> -Noah
>
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