Hi Sage,
Thanks for the reminder. I think mailing list is better for
publishing and discussing idea and potentially has more audience,
while github is more suitable for code reviewing, and cared only
by the developers. So I think for the patch to implement a new
feature, it is desirable to publish the motivation and idea behind
on the mailing list as well, hope that is not annoying :)
Cheers,
Li Wang
On 2015/6/13 10:06, Sage Weil wrote:
Hi Li,
Reviewing this now! See comments on the PR.
Just FYI, the current convention is to send kernel patches to the list,
and to use github for the userland stuff. Emails like this are helpful to
get people's attention but not strictly needed--we'll notice the PR either
way!
Thanks-
sage
On Thu, 28 May 2015, Li Wang wrote:
This patch is to do write back throttling for cache tiering,
which is similar to what the Linux kernel does for
page cache write back. The motivation and original idea are
proposed by Nick Fisk, detailed in his email as below. In our
implementation, we introduce a paramter 'cache_target_dirty_high_ratio'
(default 0.6) as the high speed threshold, while leave the
'cache_target_dirty_ratio' (default 0.4) to represent the low speed
threshold, we control the flush speed by limiting the parallelism of
flushing. The maximum parallelism under low speed is half of the
parallelism under high speed. If there is at least one PG such that
the dirty ratio beyond the high threshold, full speed mode is entered;
If there is no PG such that dirty ratio beyond the low threshold,
idle mode is entered; In other cases, slow speed mode is entered.
-------- Original Message --------
Subject: Ceph Tiering Idea
Date: Fri, 22 May 2015 16:07:46 +0100
From: Nick Fisk <nick@xxxxxxxxxx>
To: liwang@xxxxxxxxxxxxxxx
Hi,
I??ve just seen your post to the Ceph Dev Mailing list regarding
adding temperature based eviction to the cache eviction logic.
I think this is a much needed enhancement and can??t wait to test
it out once it hits the next release.
I have been testing Ceph Cache Tiering for a number of months now
and another enhancement which I think would greatly enhance the
performance would be high and low thresholds for flushing and
eviction. I have tried looking through the Ceph source, but with
my limited programming skills I was unable to make any progress
and so thought I would share my idea with you and get your thoughts.
Currently as soon as you exceed the flush/eviction threshold, Ceph
starts aggressively flushing to the base tier which impacts performance.
For long running write operations this is probably unavoidable, however
most workloads are normally quite bursty and my idea of having high and
low thresholds would hopefully improve performance where the writes
come in bursts.
When the cache tier approaches the low threshold, Ceph would start
flushing/evicting with a low priority, so performance is not affected.
If the high threshold is reached, Ceph will flush more aggressively,
similar to the current behaviour. Hopefully during the quiet periods
in-between bursts of writes, the cache would slowly be reduced down to
the low threshold meaning it is ready for the next burst.
For example:-
1TB Cache Tier
Low Dirty=0.4
High Dirty=0.6
Cache tier would contain 400GB of dirty data at idle, as dirty data
rises above 400GB, Ceph would flush with a low priority or throttled
MB/s rate.
If Cache tier raises above 600GB, Ceph will aggressively flush to keep
dirty data below 60%
The above should give you 200GB capacity of bursty writes before
performance becomes impacted
Does this make sense?
Many Thanks,
Nick
The patches:
https://github.com/ceph/ceph/pull/4792
Mingxin Liu (6):
Osd: classify flush mode into low speed and high speed modes
osd: add new field in pg_pool_t
Mon: add cache_target_dirty_high_ratio related configuration and
commands
Osd: revise agent_choose_mode() to track the flush mode
Osd: implement low speed flush
Doc: add write back throttling stuff in document and test scripts
ceph-erasure-code-corpus | 2 +-
doc/dev/cache-pool.rst | 1 +
doc/man/8/ceph.rst | 3 ++-
doc/rados/operations/cache-tiering.rst | 11 +++++++++++
doc/rados/operations/pools.rst | 19 +++++++++++++++++++
qa/workunits/cephtool/test.sh | 9 +++++++++
src/common/config_opts.h | 2 ++
src/mon/MonCommands.h | 4 ++--
src/mon/OSDMonitor.cc | 31 ++++++++++++++++++++++++++++---
src/osd/OSD.cc | 7 ++++++-
src/osd/OSD.h | 11 +++++++++++
src/osd/PG.h | 1 +
src/osd/ReplicatedPG.cc | 22 +++++++++++++++++-----
src/osd/ReplicatedPG.h | 6 +++++-
src/osd/TierAgentState.h | 6 ++++--
src/osd/osd_types.cc | 13 +++++++++++--
src/osd/osd_types.h | 3 +++
17 files changed, 133 insertions(+), 18 deletions(-)
--
1.9.1
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