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From Dongfeng Lu <dlu66...@yahoo.com>
Subject Re: (Full) compaction does not delete (all) old files
Date Wed, 01 Jun 2016 16:52:48 GMT
Alain,

Thanks for responding to my question. 

1 & 2: I think it is a bug, but as you said, maybe no one will dig it. I just hope it
has been fixed in the later versions.
3: Restarting the code does NOT remove those files. I stopped and restarted C* many times
and it did nothing. 
4: Thanks for the links. I will probably try DTCS in the near future.

A: Automatic compaction by C* does not work in a timely manner for me. I set TTL to 8 days,
and hoped that I only have data files with timestamps like within 2 weeks. However, I often
saw files created 2 months ago with 50GB in size.

In the final step of upgrade, I am suppose to run upgradesstables, which is like a compaction.
I know compaction takes a long time to run. In order to reduce the amount of time during the
actual upgrade, I ran a manual compaction to cut down the size, by 80% in my case.

B: We have tested the procedure with 2.1.11 in our DEV environment quite some time ago. Due
to priority changes, we only started applying it to production lately. By rule, I had to re-test
it if I switch to 2.1.14, and I don't see much benefits doing it.

C: Yes, I noticed the statement "When upgrading to Cassandra 2.1 all nodes must be on at least
Cassandra 2.0.7 to support rolling start." Since we are at 2.0.6, we have to migrate twice,
from 2.0.6 to 2.0.17. then to 2.1.11.

Finally, my solution was to manually delete those old files. I actually deleted them while
C* is running and did not see any errors/warnings in system.log. My guess is that those files
are not in C* metadata so C* does not know their existance.

Thanks,
Dongfeng 

    On Wednesday, June 1, 2016 6:36 AM, Alain RODRIGUEZ <arodrime@gmail.com> wrote:
 

 Hi,
About your main concern:
1. True those files should have been removed. Yet Cassandra 2.0 is no longer supported, even
more such an old version (2.0.6), so I think no one is going to dig this issue. To fix it,
upgrade will probably be enough.

I don't usually run manual compaction, and relied completely on Cassandra to automatically
do it. A couple of days ago in preparation for an upgrade to Cassandra 2.1.11, I ran a manual,
complete compaction

2. As you might know, sstables are immutable, meaning compacting, merging row shards, has
to be done somewhere else, not in place. Those -tmp- files are the result of compactions ongoing
basically. It is perfectly normal. Yet '-tmp-' files are supposed to be removed once compaction
is done.

3. Restarting the node will most probably solve your issue. To be sure to indeed free disk
space, make sure you have no snapshot of those old sstables.
4. The advantage of DTCS is that data is not mixed per age. Meaning Cassandra can drop a full
expired sstable, without compacting. It sounds like a good fit. Yet this compaction strategy
is the most recent one and some things are still being fixed. I still think it is safe to
use it. Make sure you read first: https://labs.spotify.com/2014/12/18/date-tiered-compaction/
And/Or http://www.datastax.com/dev/blog/datetieredcompactionstrategy

You also might want to have a look at https://github.com/jeffjirsa/twcs.
Some other off-topic, but maybe useful questions / info
A - Why do you need a manual compaction before upgrading? I really can't see any reason for
it.B - Why upgrading to Cassandra 2.1.14 when 2.1.14 is available and brings some more bug
fixes (compared to 2.1.11)?C - It is recommended to move to 2.0.last before going to 2.1.X.
You might run into some issue. Either make sure to test it works or go incrementally 2.0.6
--> 2.0.17 --> 2.1.14. I would probably do both. Test it and go incrementally. I would
not go with 2.0.6 --> 2.1.14 without testing it first anyway.
Hope it is all clear and that a restart will solve your issue.
C*heers,
-----------------------Alain Rodriguez - alain@thelastpickle.comFrance
The Last Pickle - Apache Cassandra Consultinghttp://www.thelastpickle.com
2016-05-17 0:06 GMT+01:00 Dongfeng Lu <dlu66061@yahoo.com>:

Forgive me if that has been answered somewhere, but I could not find a concise or clear answer.

I am using Cassandra 2.0.6 on a 3 node cluster. I don't usually run manual compaction, and
relied completely on Cassandra to automatically do it. A couple of days ago in preparation
for an upgrade to Cassandra 2.1.11, I ran a manual, complete compaction. The compaction ran
for many hours, but it did complete successfully, and the "load" in "nodetool status" dropped
80%. However, I did not see a big drop in disk usage, even after waiting for a couple of days.
There are still many old data files left on the disk. For instance, here is a list of data
files for one table.

-bash-4.1$ ls -ltr *-Data.db
-rw-r--r--  1 cassandra cassandra 36441245112 Jan 19 05:42 keyspace-event_index-jb-620839-Data.db
-rw-r--r--  1 cassandra cassandra 48117578123 Jan 25 05:17 keyspace-event_index-jb-649329-Data.db
-rw-r--r--  1 cassandra cassandra  8731574747 Jan 27 18:30 keyspace-event_index-jb-662597-Data.db
-rw-r--r--  1 cassandra cassandra   835204478 Feb  2 07:20 keyspace-event_index-jb-670851-Data.db
-rw-r--r--  1 cassandra cassandra    39496133 Feb  2 15:29 keyspace-event_index-tmp-jb-672828-Data.db
... about 110 files listed here, removed for clarity ...

-rw-r--r--  1 cassandra cassandra   149344563 May  9 20:53 keyspace-event_index-tmp-jb-827472-Data.db
-rw-r--r-- 11 cassandra cassandra 20149715779 May 15 04:18 keyspace-event_index-jb-829601-Data.db
-rw-r--r-- 11 cassandra cassandra  7153875910 May 15 11:15 keyspace-event_index-jb-830446-Data.db
-rw-r--r-- 11 cassandra cassandra  3051908121 May 16 03:08 keyspace-event_index-jb-831112-Data.db
-rw-r--r-- 11 cassandra cassandra  6109582092 May 16 06:11 keyspace-event_index-jb-831709-Data.db
-rw-r--r-- 11 cassandra cassandra  2922532233 May 16 07:14 keyspace-event_index-jb-831873-Data.db
-rw-r--r-- 11 cassandra cassandra  1766025989 May 16 08:31 keyspace-event_index-jb-832111-Data.db
-rw-r--r--  8 cassandra cassandra  2922259593 May 16 11:39 keyspace-event_index-jb-832693-Data.db
-rw-r--r--  8 cassandra cassandra  1224495235 May 16 11:50 keyspace-event_index-jb-832764-Data.db
-rw-r--r--  7 cassandra cassandra  2051385733 May 16 12:57 keyspace-event_index-jb-832975-Data.db
-rw-r--r--  6 cassandra cassandra   853824939 May 16 13:12 keyspace-event_index-jb-833100-Data.db
-rw-r--r--  5 cassandra cassandra   763243638 May 16 14:58 keyspace-event_index-jb-833203-Data.db
-rw-r--r--  3 cassandra cassandra    99076639 May 16 16:29 keyspace-event_index-jb-833222-Data.db
-rw-r--r--  2 cassandra cassandra   254935385 May 16 17:21 keyspace-event_index-jb-833233-Data.db
-rw-r--r--  2 cassandra cassandra    66006223 May 16 17:51 keyspace-event_index-jb-833238-Data.db
-rw-r--r--  1 cassandra cassandra    50204322 May 16 18:18 keyspace-event_index-jb-833243-Data.db
-rw-r--r--  2 cassandra cassandra    16078537 May 16 18:26 keyspace-event_index-jb-833244-Data.db

However, it looks to me that Cassandra knows that the first 115 files are old and are not
really used to create snapshot. Here is the newly created snapshot.

-bash-4.1$ ls -ltr snapshots/20160516-1800/*-Data.db
-rw-r--r-- 11 cassandra cassandra 20149715779 May 15 04:18 snapshots/20160516-1800/keyspace-event_index-jb-829601-Data.db
-rw-r--r-- 11 cassandra cassandra  7153875910 May 15 11:15 snapshots/20160516-1800/keyspace-event_index-jb-830446-Data.db
-rw-r--r-- 11 cassandra cassandra  3051908121 May 16 03:08 snapshots/20160516-1800/keyspace-event_index-jb-831112-Data.db
-rw-r--r-- 11 cassandra cassandra  6109582092 May 16 06:11 snapshots/20160516-1800/keyspace-event_index-jb-831709-Data.db
-rw-r--r-- 11 cassandra cassandra  2922532233 May 16 07:14 snapshots/20160516-1800/keyspace-event_index-jb-831873-Data.db
-rw-r--r-- 11 cassandra cassandra  1766025989 May 16 08:31 snapshots/20160516-1800/keyspace-event_index-jb-832111-Data.db
-rw-r--r--  8 cassandra cassandra  2922259593 May 16 11:39 snapshots/20160516-1800/keyspace-event_index-jb-832693-Data.db
-rw-r--r--  8 cassandra cassandra  1224495235 May 16 11:50 snapshots/20160516-1800/keyspace-event_index-jb-832764-Data.db
-rw-r--r--  7 cassandra cassandra  2051385733 May 16 12:57 snapshots/20160516-1800/keyspace-event_index-jb-832975-Data.db
-rw-r--r--  6 cassandra cassandra   853824939 May 16 13:12 snapshots/20160516-1800/keyspace-event_index-jb-833100-Data.db
-rw-r--r--  5 cassandra cassandra   763243638 May 16 14:58 snapshots/20160516-1800/keyspace-event_index-jb-833203-Data.db
-rw-r--r--  3 cassandra cassandra    99076639 May 16 16:29 snapshots/20160516-1800/keyspace-event_index-jb-833222-Data.db
-rw-r--r--  2 cassandra cassandra   254935385 May 16 17:21 snapshots/20160516-1800/keyspace-event_index-jb-833233-Data.db
-rw-r--r--  2 cassandra cassandra    66006223 May 16 17:51 snapshots/20160516-1800/keyspace-event_index-jb-833238-Data.db
-rw-r--r--  2 cassandra cassandra    16336415 May 16 17:59 snapshots/20160516-1800/keyspace-event_index-jb-833239-Data.db
-rw-r--r--  2 cassandra cassandra     1947026 May 16 18:00 snapshots/20160516-1800/keyspace-event_index-jb-833240-Data.db
-bash-4.1$

You can see that only files dated "May 15 04:18" or later exist in the snapshot folder.

My questions:

1. I believe Cassandra should have deleted all old 115 data files. What could have prevented
those files being deleted? What can I do to make sure old files will be deleted in future
compactions?
2. What are those files with "-tmp-"? What is the implication of their existance? Does it
mean a compaction failed?  
3. Since Cassandra knows what files are actually used, is there some utility that I can use
to delete those old files? I can delete them manually, but that would be error-prone.
4. The table uses SizeTieredCompactionStrategy, and contains data with a TTL of 8 days. Will
switching to DateTieredCompactionStrategy after upgrading to 2.1.11 offer much better compaction
performance?
Thanks,Dongfeng




  
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