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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] Commented: (CASSANDRA-1882) rate limit all background I/O
Date Mon, 24 Jan 2011 16:11:44 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-1882?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12985774#action_12985774
] 

Jonathan Ellis commented on CASSANDRA-1882:
-------------------------------------------

How is this looking, Peter?

> rate limit all background I/O
> -----------------------------
>
>                 Key: CASSANDRA-1882
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-1882
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Peter Schuller
>            Assignee: Peter Schuller
>            Priority: Minor
>             Fix For: 0.7.1
>
>
> There is a clear need to support rate limiting of all background I/O (e.g., compaction,
repair). In some cases background I/O is naturally rate limited as a result of being CPU bottlenecked,
but in all cases where the CPU is not the bottleneck, background streaming I/O is almost guaranteed
(barring a very very smart RAID controller or I/O subsystem that happens to cater extremely
well to the use case) to be detrimental to the latency and throughput of regular live traffic
(reads).
> Ways in which live traffic is negatively affected by backgrounds I/O includes:
> * Indirectly by page cache eviction (see e.g. CASSANDRA-1470).
> * Reads are directly detrimental when not otherwise limited for the usual reasons; large
continuing read requests that keep coming are battling with latency sensitive live traffic
(mostly seek bound). Mixing seek-bound latency critical with bulk streaming is a classic no-no
for I/O scheduling.
> * Writes are directly detrimental in a similar fashion.
> * But in particular, writes are more difficult still: Caching effects tend to augment
the effects because lacking any kind of fsync() or direct I/O, the operating system and/or
RAID controller tends to defer writes when possible. This often leads to a very sudden throttling
of the application when caches are filled, at which point there is potentially a huge backlog
of data to write.
> ** This may evict a lot of data from page cache since dirty buffers cannot be evicted
prior to being flushed out (though CASSANDRA-1470 and related will hopefully help here).
> ** In particular, one major reason why batter-backed RAID controllers are great is that
they have the capability to "eat" storms of writes very quickly and schedule them pretty efficiently
with respect to a concurrent continuous stream of reads. But this ability is defeated if we
just throw data at it until entirely full. Instead a rate-limited approach means that data
can be thrown at said RAID controller at a reasonable pace and it can be allowed to do its
job of limiting the impact of those writes on reads.
> I propose a mechanism whereby all such backgrounds reads are rate limited in terms of
MB/sec throughput. There would be:
> * A configuration option to state the target rate (probably a global, until there is
support for per-cf sstable placement)
> * A configuration option to state the sampling granularity. The granularity would have
to be small enough for rate limiting to be effective (i.e., the amount of I/O generated in
between each sample must be reasonably small) while large enough to not be expensive (neither
in terms of gettimeofday() type over-head, nor in terms of causing smaller writes so that
would-be streaming operations become seek bound). There would likely be a recommended value
on the order of say 5 MB, with a recommendation to multiply that with the number of disks
in the underlying device (5 MB assumes classic mechanical disks).
> Because of coarse granularity (= infrequent synchronization), there should not be a significant
overhead associated with maintaining shared global rate limiter for the Cassandra instance.

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