On 08/19/2013 11:24 AM, Gregory Farnum wrote:
On Mon, Aug 19, 2013 at 9:07 AM, Sage Weil <sage@xxxxxxxxxxx> wrote:
On Mon, 19 Aug 2013, S?bastien Han wrote:
Hi guys,
While reading a developer doc, I came across the following options:
* osd balance reads = true
* osd shed reads = true
* osd shed reads min latency
* osd shed reads min latency diff
The problem is that I can't find any of these options in config_opts.h.
These are left over from an old unimplemented experiment and were removed
a while back.
Loic Dachary also gave me a flag that he found from the code.
m->get_flags() & CEPH_OSD_FLAG_LOCALIZE_READS)
So my questions are:
* Which from the above flags are correct?
* Do balanced reads really exist in RBD?
For localized reads you want
OPTION(rbd_balance_snap_reads, OPT_BOOL, false)
OPTION(rbd_localize_snap_reads, OPT_BOOL, false)
Note that the 'localize' logic is still very primitive (it matches by IP
address). There is a blueprint to improve this:
http://wiki.ceph.com/01Planning/02Blueprints/Emperor/librados%2F%2Fobjecter%3A_smarter_localized_reads
Also, there are some issues with read/write consistency when using
localized reads because the replicas do not provide the ordering
guarantees that primaries will. See
http://tracker.ceph.com/issues/5388
At present localized reads are really only suitable for spreading the
load on write-once, read-many workloads.
To be clear, those issues don't apply to these rbd options, since they
only affect reading from snapshots, which are inherently read-only.
This is particularly useful for cloned images reading from their parent
snapshot.
Josh
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