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From "Jonathan Shook (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-9666) Provide an alternative to DTCS
Date Tue, 29 Mar 2016 19:21:25 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-9666?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15216688#comment-15216688
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Jonathan Shook commented on CASSANDRA-9666:
-------------------------------------------

There are two areas of concern that we should discuss more directly..

1. The pacing of memtable flushing on a given system can be matched up with the base window
size with DTCS, avoiding logical write amplification that can occur before the scheduling
discipline kicks in. This is not so easy when  you water down the configuration and remove
the ability to manage the fresh sstables. The benefits from time-series friendly compaction
can be had for both the newest and the oldest tables, and both are relevant here.

2. The window placement. From what I've seen, the anchoring point for whether a cell goes
into a bucket or not is different between the two approaches. To me this is fairly arbitrary
in terms of processing overhead comparisons, all else assumed close enough. However, when
trying to reconcile, shifting all of your data to a different bucket will not be a welcome
event for most users. This makes "graceful" reconciliation difficult at best.

Can we simply try to make DTCS as (perceptually) easy to use for the default case as TWCS
(perceptually) ? To me, this is more about the user entry point and understanding behavior
as designed than it is about the machinery that makes it happen.

The basic design between them has so much in common that reconciling them completely would
be mostly a shell game of parameter names as well as lobbing off some functionality that can
be complete bypassed, given the right settings.

Can we identify the functionally equivalent settings for TWCS that DTCS needs to emulate,
given proper settings (possibly including anchoring point), and then simply provide the same
simple configuration to users, without having to maintain two separate sibling compaction
strategies?

One sticking point that I've had on this suggesting in conversation is the bucketing logic
being too difficult to think about. If we were able to provide the self-same behavior for
TWCS-like configuration, the bucketing logic could be used only when the parameters require
non-uniform windows. Would that make everyone happy?






> Provide an alternative to DTCS
> ------------------------------
>
>                 Key: CASSANDRA-9666
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-9666
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Jeff Jirsa
>            Assignee: Jeff Jirsa
>             Fix For: 2.1.x, 2.2.x
>
>         Attachments: dtcs-twcs-io.png, dtcs-twcs-load.png
>
>
> DTCS is great for time series data, but it comes with caveats that make it difficult
to use in production (typical operator behaviors such as bootstrap, removenode, and repair
have MAJOR caveats as they relate to max_sstable_age_days, and hints/read repair break the
selection algorithm).
> I'm proposing an alternative, TimeWindowCompactionStrategy, that sacrifices the tiered
nature of DTCS in order to address some of DTCS' operational shortcomings. I believe it is
necessary to propose an alternative rather than simply adjusting DTCS, because it fundamentally
removes the tiered nature in order to remove the parameter max_sstable_age_days - the result
is very very different, even if it is heavily inspired by DTCS. 
> Specifically, rather than creating a number of windows of ever increasing sizes, this
strategy allows an operator to choose the window size, compact with STCS within the first
window of that size, and aggressive compact down to a single sstable once that window is no
longer current. The window size is a combination of unit (minutes, hours, days) and size (1,
etc), such that an operator can expect all data using a block of that size to be compacted
together (that is, if your unit is hours, and size is 6, you will create roughly 4 sstables
per day, each one containing roughly 6 hours of data). 
> The result addresses a number of the problems with DateTieredCompactionStrategy:
> - At the present time, DTCS’s first window is compacted using an unusual selection
criteria, which prefers files with earlier timestamps, but ignores sizes. In TimeWindowCompactionStrategy,
the first window data will be compacted with the well tested, fast, reliable STCS. All STCS
options can be passed to TimeWindowCompactionStrategy to configure the first window’s compaction
behavior.
> - HintedHandoff may put old data in new sstables, but it will have little impact other
than slightly reduced efficiency (sstables will cover a wider range, but the old timestamps
will not impact sstable selection criteria during compaction)
> - ReadRepair may put old data in new sstables, but it will have little impact other than
slightly reduced efficiency (sstables will cover a wider range, but the old timestamps will
not impact sstable selection criteria during compaction)
> - Small, old sstables resulting from streams of any kind will be swiftly and aggressively
compacted with the other sstables matching their similar maxTimestamp, without causing sstables
in neighboring windows to grow in size.
> - The configuration options are explicit and straightforward - the tuning parameters
leave little room for error. The window is set in common, easily understandable terms such
as “12 hours”, “1 Day”, “30 days”. The minute/hour/day options are granular enough
for users keeping data for hours, and users keeping data for years. 
> - There is no explicitly configurable max sstable age, though sstables will naturally
stop compacting once new data is written in that window. 
> - Streaming operations can create sstables with old timestamps, and they'll naturally
be joined together with sstables in the same time bucket. This is true for bootstrap/repair/sstableloader/removenode.

> - It remains true that if old data and new data is written into the memtable at the same
time, the resulting sstables will be treated as if they were new sstables, however, that no
longer negatively impacts the compaction strategy’s selection criteria for older windows.

> Patch provided for : 
> - 2.1: https://github.com/jeffjirsa/cassandra/commits/twcs-2.1 
> - 2.2: https://github.com/jeffjirsa/cassandra/commits/twcs-2.2
> - trunk (post-8099):  https://github.com/jeffjirsa/cassandra/commits/twcs 
> Rebased, force-pushed July 18, with bug fixes for estimated pending compactions and potential
starvation if more than min_threshold tables existed in current window but STCS did not consider
them viable candidates
> Rebased, force-pushed Aug 20 to bring in relevant logic from CASSANDRA-9882



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