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From "Hiller, Dean" <>
Subject Re: Size Tiered -> Leveled Compaction
Date Fri, 08 Mar 2013 18:50:06 GMT
+1  (I would love to know this info).


From: Wei Zhu <<>>
Reply-To: "<>" <<>>,
Wei Zhu <<>>
Date: Friday, March 8, 2013 11:11 AM
To: "<>" <<>>
Subject: Re: Size Tiered -> Leveled Compaction

I have the same wonder.
We started with the default 5M and the compaction after repair takes too long on 200G node,
so we increase the size to 10M sort of arbitrarily since there is not much documentation around
it. Our tech op team still thinks there are too many files in one directory. To fulfill the
guidelines from them (don't remember the exact number, but something in the range of 50K files),
we will need to increase the size to around 50M. I think the latency of  opening one file
is not impacted much by the number of files in one directory for the modern file system. But
"ls" and other operations suffer.

Anyway, I asked about the side effect of the bigger SSTable in IRC, someone was mentioning
during read, C* reads the whole SSTable from disk in order to access the row which causes
more disk IO compared with the smaller SSTable. I don't know enough about the internal of
the Cassandra, not sure whether it's the case or not. If that is the case (with question mark)
, the SSTable or the row is kept in the memory? Hope someone can confirm the theory here.
Or I have to dig in to the source code to find it.

Another concern is during repair, does it stream the whole SSTable or the partial of it when
mismatch is detected? I see the claim for both, can someone please confirm also?

The last thing is the effectiveness of the parallel LCS on 1.2. It takes quite some time for
the compaction to finish after repair for LCS for 1.1.X. Both CPU and disk Util is low during
the compaction which means LCS doesn't fully utilized resource.  It will make the life easier
if the issue is addressed in 1.2.

Bottom line is that there is not much documentation/guideline/successful story around LCS
although it sounds beautiful on paper.

From: Alain RODRIGUEZ <<>>
Cc: Wei Zhu <<>>
Sent: Friday, March 8, 2013 1:25 AM
Subject: Re: Size Tiered -> Leveled Compaction

I'm still wondering about how to chose the size of the sstable under LCS. Defaul is 5MB, people
use to configure it to 10MB and now you configure it at 128MB. What are the benefits or inconveniants
of a very small size (let's say 5 MB) vs big size (like 128MB) ?


2013/3/8 Al Tobey <<>>
We saw the exactly the same thing as Wei Zhu, > 100k tables in a directory causing all
kinds of issues.  We're running 128MiB ssTables with LCS and have disabled compaction throttling.
 128MiB was chosen to get file counts under control and reduce the number of files C* has
to manage & search. I just looked and a ~250GiB node is using about 10,000 files, which
is quite manageable.  This configuration is running smoothly in production under mixed read/write

We're on RAID0 across 6 15k drives per machine. When we migrated data to this cluster we were
pushing well over 26k/s+ inserts with CL_QUORUM. With compaction throttling enabled at any
rate it just couldn't keep up. With throttling off, it runs smoothly and does not appear to
have an impact on our applications, so we always leave it off, even in EC2.  An 8GiB heap
is too small for this config on 1.1. YMMV.

-Al Tobey

On Thu, Feb 14, 2013 at 12:51 PM, Wei Zhu <<>>
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 <<>>

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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