Osd: write back throttling for cache tiering

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 <[email protected]>
To: [email protected]

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(-)

Signature · 6 lines

1.9.1

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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

Quoted 112 lines · Li Wang

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 <[email protected]>
To: [email protected]

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(-)

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