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From "Hiller, Dean" <>
Subject Re: Size Tiered -> Leveled Compaction
Date Mon, 25 Feb 2013 14:47:41 GMT
Sweet, thanks for the info.

From: Alain RODRIGUEZ <<>>
Reply-To: "<>" <<>>
Date: Monday, February 25, 2013 7:41 AM
To: "<>" <<>>
Subject: Re: Size Tiered -> Leveled Compaction

"After running a major compaction, automatic minor compactions are no longer triggered,"

... Because of the size difference between the big sstable generated and the new sstable flushed/compacted.
Compactions are not stopped, they are just "no longer triggered" for a while.

"frequently requiring you to manually run major compactions on a routine basis"

... In order to keep a good read latency. If you don't run compaction periodically and you
have some row update, you will have an increasing amount of rows spread across various sstable.
But my guess is that if you have no delete, no update and no ttl but only write once row,
you may keep this big table uncompacted for as long as you want without any read performance

I think the documentation just don't go deep enough in the explanation, or maybe this information
already exists somewhere else in the documentation.

Wait a confirmation of an expert, I am just an humble user.


2013/2/25 Hiller, Dean <<>>
So what you are saying is this documentation is not quite accurate then….(I am more confused
between your statement and the documentation now)

Which says "After running a major compaction, automatic minor compactions are no longer triggered,
frequently requiring you to manually run major compactions on a routine basis"

Which implied that you have to keep running major compactions and minor compactions are not
kicking in anymore :( :( and we(my project) want minor compactions to continue.


From: Alain RODRIGUEZ <<><<>>>
Reply-To: "<><<>>"
Date: Monday, February 25, 2013 7:15 AM
To: "<><<>>"
Subject: Re: Size Tiered -> Leveled Compaction

"I am confused.  I thought running compact turns off the minor compactions and users are actually
supposed to run upgradesstables????  (maybe I am on old documentation?)"

Well, that's not true. What happens is that compaction use sstables with an aproximate same
size. So if you run a major compaction on a 10GB CF, you have almost no chance of getting
that (big) sstable compacted again. You will have to wait for other sstables to reach this
size or run an other major compaction.

But anyways, this doesn't apply here because we are speaking of LCS (leveled compaction strategy),
which runs differently from the traditional STC (sized tier compaction).

Not sure about it, but you may run upgradesstable or compaction to rebuild your sstable after
switching from STC  to LCS, I mean both methods trigger an initialization of LCS on old sstables.


2013/2/25 Hiller, Dean <<><<>>>
I am confused.  I thought running compact turns off the minor compactions and users are actually
supposed to run upgradesstables????  (maybe I am on old documentation?)

Can someone verify that?


From: Michael Theroux <<><<>><<><<>>>>
Reply-To: "<><<>><<><<>>>"
Date: Sunday, February 24, 2013 7:45 PM
To: "<><<>><<><<>>>"
Subject: Re: Size Tiered -> Leveled Compaction


Thanks for the response.  I think I speak for many Cassandra users when I say we greatly appreciate
your help with our questions and issues.  For the specific bug I mentioned, I found this comment

"Automatic fixing of overlapping leveled sstables (CASSANDRA-4644)"

Although I had difficulty putting 2 and 2 together from the comments in 4644 (it mentioned
being fixed in 1.1.6, but also being not reproducible).

We converted two column families yesterday (two we believe would be particularly well suited
for Leveled Compaction).  We have two more to convert, but those will wait until next weekend.
 So far no issues, and, we've seen some positive results.

To help answer some of my own questions I posed in this thread, and others have expressed
interest in knowing, the steps we followed were:

1) Perform the proper alter table command:

ALTER TABLE X WITH compaction_strategy_class='LeveledCompactionStrategy' AND  compaction_strategy_options:sstable_size_in_mb=10;

2) Ran compact on all nodes

nodetool compact <keyspace> X

We converted one column family at a time, and temporarily disabled some maintenance activities
we perform to decrease load while we converted column families, as the compaction was resource
heavy and I didn't wish to interfere with our operational activities as much as possible.
   In our case, the compaction after altering the schema, took about an hour and a half.

Thus far, it appears everything worked without a hitch.  I chose 10 mb for the SSTABLE size,
based on Wei's feedback (who's data size is on-par with ours), and other tid-bits I found
through searching.  Based on issues people have reported in the relatively distant past. I
made sure that we've been handling the compaction load properly, and I've run test repairs
on the specific tables we converted.  We also tested restarting a node after the conversion.

Again, I believe the tables we converted were particularly well suited for Leveled Compaction.
 These particular column families were situations where reads outstripped writes by an order
of magnitude or two.

So far, our results have been very positive.  We've seen a greater than 50% reduction in read
I/O, and a large improvement in performance for some activities.  We've also seen an improvement
in memory utilization.  I imagine other's mileage may vary.

If everything is stable over the next week, we will convert the last two tables we are considering
for Leveled Compaction.

Thanks again!

On Feb 24, 2013, at 8:56 PM, aaron morton wrote:

If you did not use LCS until after the upgrade to 1.1.9 I think you are ok.

If in doubt the steps here look like they helped


Aaron Morton
Freelance Cassandra Developer
New Zealand


On 23/02/2013, at 6:56 AM, Mike <<><<>><<><<>>>>


Still doing research before we potentially move one of our column families from Size Tiered->Leveled
compaction this weekend.  I was doing some research around some of the bugs that were filed
against leveled compaction in Cassandra and I found this:

The bug mentions:

"You need to run the offline scrub (bin/sstablescrub) to fix the sstable overlapping problem
from early 1.1 releases. (Running with -m to just check for overlaps between sstables should
be fine, since you already scrubbed online which will catch out-of-order within an sstable.)"

We recently upgraded from 1.1.2 to 1.1.9.

Does anyone know if an offline scrub is recommended to be performed when switching from STCS->LCS
after upgrading from 1.1.2?

Any insight would be appreciated,

On 2/17/2013 8:57 PM, Wei Zhu wrote:
We doubled the SStable size to 10M. It still generates a lot of SSTable and we don't see much
difference of the read latency.  We are able to finish the compactions after repair within
serveral hours. We will increase the SSTable size again if we feel the number of SSTable hurts
the performance.

----- Original Message -----
From: "Mike" <<><<>><<><<>>>>
Sent: Sunday, February 17, 2013 4:50:40 AM
Subject: Re: Size Tiered -> Leveled Compaction

Hello Wei,

First thanks for this response.

Out of curiosity, what SSTable size did you choose for your usecase, and what made you decide
on that number?


On 2/14/2013 3:51 PM, Wei Zhu wrote:

I haven't tried to switch compaction strategy. We started with LCS.

For us, after massive data imports (5000 w/seconds for 6 days), the first repair is painful
since there is quite some data inconsistency. For 150G nodes, repair brought in about 30 G
and created thousands of pending compactions. It took almost a day to clear those. Just be
prepared LCS is really slow in 1.1.X. System performance degrades during that time since reads
could go to more SSTable, we see 20 SSTable lookup for one read.. (We tried everything we
can and couldn't speed it up. I think it's single threaded.... and it's not recommended to
turn on multithread compaction. We even tried that, it didn't help )There is parallel LCS
in 1.2 which is supposed to alleviate the pain. Haven't upgraded yet, hope it works:)

Since our cluster is not write intensive, only 100 w/seconds. I don't see any pending compactions
during regular operation.

One thing worth mentioning is the size of the SSTable, default is 5M which is kind of small
for 200G (all in one CF) data set, and we are on SSD. It more than 150K files in one directory.
(200G/5M = 40K SSTable and each SSTable creates 4 files on disk) You might want to watch that
and decide the SSTable size.

By the way, there is no concept of Major compaction for LCS. Just for fun, you can look at
a file called $CFName.json in your data directory and it tells you the SSTable distribution
among different levels.


From: Charles Brophy <<><<>><<><<>>>>
Sent: Thursday, February 14, 2013 8:29 AM
Subject: Re: Size Tiered -> Leveled Compaction

I second these questions: we've been looking into changing some of our CFs to use leveled
compaction as well. If anybody here has the wisdom to answer them it would be of wonderful


On Wed, Feb 13, 2013 at 7:50 AM, Mike <<><<>><<><<>>>
> wrote:


I'm investigating the transition of some of our column families from Size Tiered -> Leveled
Compaction. I believe we have some high-read-load column families that would benefit tremendously.

I've stood up a test DB Node to investigate the transition. I successfully alter the column
family, and I immediately noticed a large number (1000+) pending compaction tasks become available,
but no compaction get executed.

I tried running "nodetool sstableupgrade" on the column family, and the compaction tasks don't

I also notice no changes to the size and distribution of the existing SSTables.

I then run a major compaction on the column family. All pending compaction tasks get run,
and the SSTables have a distribution that I would expect from LeveledCompaction (lots and
lots of 10MB files).

Couple of questions:

1) Is a major compaction required to transition from size-tiered to leveled compaction?
2) Are major compactions as much of a concern for LeveledCompaction as their are for Size

All the documentation I found concerning transitioning from Size Tiered to Level compaction
discuss the alter table cql command, but I haven't found too much on what else needs to be
done after the schema change.

I did these tests with Cassandra 1.1.9.


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