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From "Lucas de Souza Santos (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-9666) Provide an alternative to DTCS
Date Wed, 13 Apr 2016 21:23:26 GMT


Lucas de Souza Santos commented on CASSANDRA-9666:

I have been working for the biggest online news in Brazil and we are using/building a timeseries
system with cassandra as persistence.
Since the end of last year we decided to use DTCS. This timeseries implementation is rolling
to production to substitute legacy cacti.

Right now the cluster is composed by 10 Dell PE R6XX, some 610 and some 620. All with SAS
discs, 8 cpus and 32GB of RAM, Linux Centos 6, kernel 2.

Since Jan 20 2016 we are running cassandra21-2.1.12 over JRE 7. At that point we were just
doing some tests, receiving ~140k points/minute. The cluster was fine and using STCS (the
default) as compaction strategy.

At the end of February I changed to DTCS and we doubled the load, passing to around 200k points/minute.
A week after, we saw the cpu load growing up, together with disc space and memory. First we
thought it was us using more, so we built some dashboards to visualize the data.

About 3 weeks ago the cluster started to get some timeouts and we lost a node at least two
times, a reboot was needed to get the node back.

Things I have done trying to fix/improve the cluster:

Upgraded jre7 to jdk8, configured GC G1, altered memtable_cleanup_threshold to 0.10 (was using
0.20, getting this value high made the problem worst).

Changed all applications using cassandra to use consistency ONE because GC pause was putting
nodes out of the cluster and we were receiving a lot of timeouts.
After those changes the cluster was better to use but we were not confident in growing the
number of requests. Last week I noticed a problem when restarting any node, it took at lest
15 minutes, sometimes 30 minutes just to load/open sstables. I checked the data on disc and
saw that cassandra created more than 10 million sstables. I couldn't do a simple "ls" in any
datadir (I have 14 keyspaces).

> Provide an alternative to DTCS
> ------------------------------
>                 Key: CASSANDRA-9666
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Jeff Jirsa
>            Assignee: Jeff Jirsa
>             Fix For: 2.1.x, 2.2.x
>         Attachments: dashboard-DTCS_to_TWCS.png, 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
> - 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: 
> - 2.2:
> - trunk (post-8099): 
> 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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