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From Mark Rose <markr...@markrose.ca>
Subject Re: STCS Compaction with wide rows & TTL'd data
Date Fri, 02 Sep 2016 16:33:34 GMT
Hi Kevin,

The tombstones will live in an sstable until it gets compacted. Do you
have a lot of pending compactions? If so, increasing the number of
parallel compactors may help. You may also be able to tun the STCS
parameters. Here's a good explanation of how it works:
https://shrikantbang.wordpress.com/2014/04/22/size-tiered-compaction-strategy-in-apache-cassandra/

Anyway, LCS would probably be a better fit for your use case. LCS
would help with eliminating tombstones, but it may also result in
dramatically higher CPU usage for compaction. If LCS compaction can
keep up, in addition to getting ride of tombstones faster, LCS should
reduce the number of sstables that must be read to return the row and
have a positive impact on read latency. STCS is a bad fit for rows
that are updated frequently (which includes rows with TTL'ed data).

Also, you may have an error in your application design. OAuth Access
Tokens are designed to have a very short lifetime of seconds or
minutes. On access token expiry, a Refresh Token should be used to get
a new access token. A long-lived access token is a dangerous thing as
there is no way to disable it (refresh tokens should be disabled to
prevent the creation of new access tokens).

-Mark

On Thu, Sep 1, 2016 at 3:53 AM, Kevin O'Connor <kevin@reddit.com> wrote:
> We're running C* 1.2.11 and have two CFs, one called OAuth2AccessToken and
> one OAuth2AccessTokensByUser. OAuth2AccessToken has the token as the row
> key, and the columns are some data about the OAuth token. There's a TTL set
> on it, usually 3600, but can be higher (up to 1 month).
> OAuth2AccessTokensByUser has the user as the row key, and then all of the
> user's token identifiers as column values. Each of the column values has a
> TTL that is set to the same as the access token it corresponds to.
>
> The OAuth2AccessToken CF takes up around ~6 GB on disk, whereas the
> OAuth2AccessTokensByUser CF takes around ~110 GB. If I use sstablemetadata,
> I can see the droppable tombstones ratio is around 90% for the larger
> sstables.
>
> My question is - why aren't these tombstones getting compacted away? I'm
> guessing that it's because we use STCS and the large sstables that have
> built up over time are never considered for compaction. Would LCS be a
> better fit for the issue of trying to keep the tombstones in check?
>
> I've also tried forceUserDefinedCompaction via JMX on some of the largest
> sstables and it just creates a new sstable of the exact same size, which was
> pretty surprising. Why would this explicit request to compact an sstable not
> remove tombstones?
>
> Thanks!
>
> Kevin

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