[PATCH] Osd: write back throttling for cache tiering

[Li Wang <liwang@ubuntukylin.com>]

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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@fisk.me.uk>
To: 	liwang@ubuntukylin.com

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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