On 5/7/2014 11:53 AM, Gregory Farnum wrote:
On Wed, May 7, 2014 at 8:44 AM, Dan van der Ster
<daniel.vanderster@xxxxxxx> wrote:
Hi,
Sage Weil wrote:
* *Primary affinity*: Ceph now has the ability to skew selection of
OSDs as the "primary" copy, which allows the read workload to be
cheaply skewed away from parts of the cluster without migrating any
data.
Can you please elaborate a bit on this one? I found the blueprint [1] but
still don't quite understand how it works. Does this only change the crush
calculation for reads? i.e writes still go to the usual primary, but reads
are distributed across the replicas? If so, does this change the consistency
model in any way.
It changes the calculation of who becomes the primary, and that
primary serves both reads and writes. In slightly more depth:
Previously, the primary has always been the first OSD chosen as a
member of the PG.
For erasure coding, we added the ability to specify a primary
independent of the selection ordering. This was part of a broad set of
changes to prevent moving the EC "shards" around between different
members of the PG, and means that the primary might be the second OSD
in the PG, or the fourth.
Once this work existed, we realized that it might be useful in other
cases, because primaries get more of the work for their PG (serving
all reads, coordinating writes).
So we added the ability to specify a "primary affinity", which is like
the CRUSH weights but only impacts whether you become the primary. So
if you have 3 OSDs that each have primary affinity = 1, it will behave
as normal. If two have primary affinity = 0, the remaining OSD will be
the primary. Etc.
Is it possible (and/or advisable) to set primary affinity low while
backfilling / recovering an OSD in an effort to prevent unnecessary slow
reads that could be directed to less busy replicas? I suppose if the
cost of setting/unsetting primary affinity is low and clients are
starved for reads during backfill/recovery from the osd in question, it
could be a win.
Perhaps the workflow for maintenance on osd.0 would be something like:
- Stop osd.0, do some maintenance on osd.0
- Read primary affinity of osd.0, store it for later
- Set primary affinity on osd.0 to 0
- Start osd.0
- Enjoy a better backfill/recovery experience. RBD clients happier.
- Reset primary affinity on osd.0 to previous value
If the cost of setting primary affinity is low enough, perhaps this
strategy could be automated by the ceph daemons.
Thanks,
Mike Dawson
-Greg
Software Engineer #42 @ http://inktank.com | http://ceph.com
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