[Qemu-devel] GSOC Report: Moving I/O throttling and write notifiers into block filter drivers

* [Qemu-devel] GSOC Report: Moving I/O throttling and write notifiers into block filter drivers
@ 2017-08-28 14:57 Manos Pitsidianakis
  2017-08-29  8:51 ` Stefan Hajnoczi
  2017-09-04  9:03 ` Stefan Hajnoczi
  0 siblings, 2 replies; 4+ messages in thread
From: Manos Pitsidianakis @ 2017-08-28 14:57 UTC (permalink / raw)
  To: qemu-devel; +Cc: Alberto Garcia, Stefan Hajnoczi, Kevin Wolf

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This is a GSOC project summary required for the project's final 
submission. As part of GSOC 2017, I took the project of moving two hard 
coded block layer features into filter drivers. I/O Throttling is 
implemented in block/throttle.c and before write notifiers are split 
into a driver for each user of the before write notifier API: 
block/backup.c and block/write-threshold.c. Furthermore, work began on 
block-insert-node and block-remove-node commands for the QMP interface 
to allow runtime insertion and removal of filter drivers. [0]

A lot of thanks to my mentors for their help: Alberto Garcia, Stefan 
Hajnoczi and Kevin Wolf.

Terms
=====

The BlockBackend struct (block/block-backend.c) represents the backend part 
of a storage device that a VM sees in the QEMU environment. The BlockBackend 
has the responsibility to forward I/O requests from the VM down to the 
actual underlying storage; a network block device, a qcow2 image etc.

In order to allow for polymorphic storage, a BlockBackend forwards the 
requests to an acyclic graph in which the leaves are the terminal I/O 
destination, a file or network connection. The BlockDriverState struct 
represents a node in this graph, and each node is governed by a specific 
driver. Above the leaf-nodes we have format drivers that translate requests 
for each format (ie a qcow2 driver, or a raw driver). Backing files are 
implemented as chains of nodes that forward read requests to their 
children but keep write requests to themselves. This setup allows 
different node drivers to intercept requests before they reach their 
destination by being inserted into points of interest in the graph. We 
call these block filter drivers.

An existing filter driver for example is block/blkverify.c which compares 
two children for content parity and reports content mismatches.

I/O Throttling
==============

I/O throttling is done by intercepting I/O requests and throttling them 
based on the configured limits (docs/throttle.txt). The interface was 
refactored into the throttle driver [1] while the throttling primitives 
were left unchanged. The already existing interface of setting limits on 
a BlockBackend device is simulated [2] by inserting a hidden to the user 
throttle filter node at the root of the BlockBackend with the user's set 
limits. Implicitly created filter nodes is not a good solution since 
some of the QEMU internals are written without considering filter nodes.  
Some patches in the throttle-remove-legacy branch are dedicated to 
changing existing behaviour to match the new concept of implicit 
filters. In the future management tools should be expected to explicitly 
add and remove filter nodes like throttle (except for transient block 
job filters which may remain implicit) and there should be no surprises 
about the state of the block layer graph for the user.

Throttle groups are categories of drives sharing the same limits with a 
round-robin algorithm. Additional effort was spent on making throttle groups 
easier to configure by turning them into a separately creatable object (with 
-object syntax on command line invocation or object-add in QMP). Their 
properties can be set with 'qom-set' commands and retrieved with 'qom-get'.

Write Notifiers
===============

While a backup block job is running, it is important to have knowledge of 
writes to the relevant image. Before write notifiers pass the write requests 
to the backup job to perform copy on write on the target image with the new 
data. Currently this is done on the BlockBackend level. Other block jobs 
(commit/mirror) already create implicit nodes in the BDS chain and this 
approach was copied and a backup filter driver was created [3], internal to 
block/backup.c

The write-threshold feature once enabled via QMP, watches passing write 
requests and compares them to a user-given threshold offset. When that 
threshold is exceeded an event is delivered to the user and the feature 
disables itself. This is used for management tools that need to know when 
they have to resize their images. Like backup, this was done in the 
BlockBackend level. However it wasn't easy to replace the existing interface 
with an implicit filter node like in throttling, so only a separate driver 
was created [4] in block/write-threshold.c. Like other filter drivers, it 
can be inserted on runtime and removed once it delivers the event and is 
spent and should be removed or replaced.

Branches / Patches
==================

The 'throttle' and 'throttle-remove-legacy' patches should be merged soon 
after master unfreezes from the 2.10 release. The rest of the patch series 
are in final stages of review on qemu-devel except for block-insert-node 
which is an RFC [5].

Already merged patches in 2.10
  https://github.com/qemu/qemu/commits/v2.10.0-rc4?author=epilys
Already merged patches for 2.11
  https://www.mail-archive.com/qemu-devel@nongnu.org/msg470461.html

[0] [insert-node] block-insert-node and block-remove-node commands
  https://github.com/epilys/qemu/commits/insert-node?author=epilys

[1] [throttle] add throttle filter driver
  https://github.com/epilys/qemu/commits/throttle?author=epilys
  Message-ID: <20170825132028.6184-1-el13635@mail.ntua.gr>
  https://www.mail-archive.com/qemu-devel@nongnu.org/msg476047.html

[2] [throttle-remove-legacy] remove legacy throttling interface
  https://github.com/epilys/qemu/commits/throttle-remove-legacy?author=epilys

[3] [4] [notify] https://github.com/epilys/qemu/commits/notify?author=epilys

[5] block-insert-node RFC
  Message-ID: <20170815074513.9055-1-el13635@mail.ntua.gr>
  https://www.mail-archive.com/qemu-devel@nongnu.org/msg473619.html

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