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Posted to user@cassandra.apache.org by cem <ca...@gmail.com> on 2013/05/21 22:43:47 UTC
Cassandra 1.2 TTL histogram problem
Hi all,
I have a question about ticket
https://issues.apache.org/jira/browse/CASSANDRA-3442
Why does Cassandra single table compaction skips the keys that are in the
other sstables? Please correct if I am wrong.
I also dont understand why we have this line in worthDroppingTombstones
method:
double remainingColumnsRatio = ((double) columns) / (sstable.
getEstimatedColumnCount<http://grepcode.com/file/repo1.maven.org/maven2/org.apache.cassandra/cassandra-all/1.2.2/org/apache/cassandra/io/sstable/SSTableReader.java#SSTableReader.getEstimatedColumnCount%28%29>
().count<http://grepcode.com/file/repo1.maven.org/maven2/org.apache.cassandra/cassandra-all/1.2.2/org/apache/cassandra/utils/EstimatedHistogram.java#EstimatedHistogram.count%28%29>
() * sstable.getEstimatedColumnCount<http://grepcode.com/file/repo1.maven.org/maven2/org.apache.cassandra/cassandra-all/1.2.2/org/apache/cassandra/io/sstable/SSTableReader.java#SSTableReader.getEstimatedColumnCount%28%29>
().mean<http://grepcode.com/file/repo1.maven.org/maven2/org.apache.cassandra/cassandra-all/1.2.2/org/apache/cassandra/utils/EstimatedHistogram.java#EstimatedHistogram.mean%28%29>
());
remainingColumnsRatio is always *0 *in my case and the droppableRatio
is *0.9.
Cassandra skips all sstables which are already expired.*
This line was introduced by
https://issues.apache.org/jira/browse/CASSANDRA-4022.
Best Regards,
Cem
Re: Cassandra 1.2 TTL histogram problem
Posted by Yuki Morishita <mo...@gmail.com>.
> Are you sure that it is a good idea to estimate remainingKeys like that?
Since we don't want to scan every row to check overlap and cause heavy
IO automatically, the method can only do the best-effort type of
calculation.
In your case, try running user defined compaction on that sstable
file. It goes through every row and remove tombstones when droppable.
On Wed, May 22, 2013 at 11:48 AM, cem <ca...@gmail.com> wrote:
> Thanks for the answer.
>
> It means that if we use randompartioner it will be very difficult to find a
> sstable without any overlap.
>
> Let me give you an example from my test.
>
> I have ~50 sstables in total and an sstable with droppable ratio 0.9. I use
> GUID for key and only insert (no update -delete) so I dont expect a key in
> different sstables.
>
> I put extra logging to AbstractCompactionStrategy to see the
> overlaps.size() and keys and remainingKeys:
>
> overlaps.size() is around 30, number of keys for that sstable is around 5 M
> and remainingKeys is always 0.
>
> Are you sure that it is a good idea to estimate remainingKeys like that?
>
> Best Regards,
> Cem
>
>
>
> On Wed, May 22, 2013 at 5:58 PM, Yuki Morishita <mo...@gmail.com> wrote:
>>
>> > Can method calculate non-overlapping keys as overlapping?
>>
>> Yes.
>> And randomized keys don't matter here since sstables are sorted by
>> "token" calculated from key by your partitioner, and the method uses
>> sstable's min/max token to estimate overlap.
>>
>> On Tue, May 21, 2013 at 4:43 PM, cem <ca...@gmail.com> wrote:
>> > Thank you very much for the swift answer.
>> >
>> > I have one more question about the second part. Can method calculate
>> > non-overlapping keys as overlapping? I mean it uses max and min tokens
>> > and
>> > column count. They can be very close to each other if random keys are
>> > used.
>> >
>> > In my use case I generate a GUID for each key and send a single write
>> > request.
>> >
>> > Cem
>> >
>> > On Tue, May 21, 2013 at 11:13 PM, Yuki Morishita <mo...@gmail.com>
>> > wrote:
>> >>
>> >> > Why does Cassandra single table compaction skips the keys that are in
>> >> > the other sstables?
>> >>
>> >> because we don't want to resurrect deleted columns. Say, sstable A has
>> >> the column with timestamp 1, and sstable B has the same column which
>> >> deleted at timestamp 2. Then if we purge that column only from sstable
>> >> B, we would see the column with timestamp 1 again.
>> >>
>> >> > I also dont understand why we have this line in
>> >> > worthDroppingTombstones
>> >> > method
>> >>
>> >> What the method is trying to do is to "guess" how many columns that
>> >> are not in the rows that don't overlap, without actually going through
>> >> every rows in the sstable. We have statistics like column count
>> >> histogram, min and max row token for every sstables, we use those in
>> >> the method to estimate how many columns the two sstables overlap.
>> >> You may have remainingColumnsRatio of 0 when the two sstables overlap
>> >> almost entirely.
>> >>
>> >>
>> >> On Tue, May 21, 2013 at 3:43 PM, cem <ca...@gmail.com> wrote:
>> >> > Hi all,
>> >> >
>> >> > I have a question about ticket
>> >> > https://issues.apache.org/jira/browse/CASSANDRA-3442
>> >> >
>> >> > Why does Cassandra single table compaction skips the keys that are in
>> >> > the
>> >> > other sstables? Please correct if I am wrong.
>> >> >
>> >> > I also dont understand why we have this line in
>> >> > worthDroppingTombstones
>> >> > method:
>> >> >
>> >> > double remainingColumnsRatio = ((double) columns) /
>> >> > (sstable.getEstimatedColumnCount().count() *
>> >> > sstable.getEstimatedColumnCount().mean());
>> >> >
>> >> > remainingColumnsRatio is always 0 in my case and the droppableRatio
>> >> > is
>> >> > 0.9. Cassandra skips all sstables which are already expired.
>> >> >
>> >> > This line was introduced by
>> >> > https://issues.apache.org/jira/browse/CASSANDRA-4022.
>> >> >
>> >> > Best Regards,
>> >> > Cem
>> >>
>> >>
>> >>
>> >> --
>> >> Yuki Morishita
>> >> t:yukim (http://twitter.com/yukim)
>> >
>> >
>>
>>
>>
>> --
>> Yuki Morishita
>> t:yukim (http://twitter.com/yukim)
>
>
--
Yuki Morishita
t:yukim (http://twitter.com/yukim)
Re: Cassandra 1.2 TTL histogram problem
Posted by cem <ca...@gmail.com>.
Thanks for the answer.
It means that if we use randompartioner it will be very difficult to find
a sstable without any overlap.
Let me give you an example from my test.
I have ~50 sstables in total and an sstable with droppable ratio 0.9. I use
GUID for key and only insert (no update -delete) so I dont expect a key
in different sstables.
I put extra logging to AbstractCompactionStrategy to see
the overlaps.size() and keys and remainingKeys:
overlaps.size() is around 30, number of keys for that sstable is around 5 M
and remainingKeys is always 0.
Are you sure that it is a good idea to estimate remainingKeys like that?
Best Regards,
Cem
On Wed, May 22, 2013 at 5:58 PM, Yuki Morishita <mo...@gmail.com> wrote:
> > Can method calculate non-overlapping keys as overlapping?
>
> Yes.
> And randomized keys don't matter here since sstables are sorted by
> "token" calculated from key by your partitioner, and the method uses
> sstable's min/max token to estimate overlap.
>
> On Tue, May 21, 2013 at 4:43 PM, cem <ca...@gmail.com> wrote:
> > Thank you very much for the swift answer.
> >
> > I have one more question about the second part. Can method calculate
> > non-overlapping keys as overlapping? I mean it uses max and min tokens
> and
> > column count. They can be very close to each other if random keys are
> used.
> >
> > In my use case I generate a GUID for each key and send a single write
> > request.
> >
> > Cem
> >
> > On Tue, May 21, 2013 at 11:13 PM, Yuki Morishita <mo...@gmail.com>
> wrote:
> >>
> >> > Why does Cassandra single table compaction skips the keys that are in
> >> > the other sstables?
> >>
> >> because we don't want to resurrect deleted columns. Say, sstable A has
> >> the column with timestamp 1, and sstable B has the same column which
> >> deleted at timestamp 2. Then if we purge that column only from sstable
> >> B, we would see the column with timestamp 1 again.
> >>
> >> > I also dont understand why we have this line in
> worthDroppingTombstones
> >> > method
> >>
> >> What the method is trying to do is to "guess" how many columns that
> >> are not in the rows that don't overlap, without actually going through
> >> every rows in the sstable. We have statistics like column count
> >> histogram, min and max row token for every sstables, we use those in
> >> the method to estimate how many columns the two sstables overlap.
> >> You may have remainingColumnsRatio of 0 when the two sstables overlap
> >> almost entirely.
> >>
> >>
> >> On Tue, May 21, 2013 at 3:43 PM, cem <ca...@gmail.com> wrote:
> >> > Hi all,
> >> >
> >> > I have a question about ticket
> >> > https://issues.apache.org/jira/browse/CASSANDRA-3442
> >> >
> >> > Why does Cassandra single table compaction skips the keys that are in
> >> > the
> >> > other sstables? Please correct if I am wrong.
> >> >
> >> > I also dont understand why we have this line in
> worthDroppingTombstones
> >> > method:
> >> >
> >> > double remainingColumnsRatio = ((double) columns) /
> >> > (sstable.getEstimatedColumnCount().count() *
> >> > sstable.getEstimatedColumnCount().mean());
> >> >
> >> > remainingColumnsRatio is always 0 in my case and the droppableRatio
> is
> >> > 0.9. Cassandra skips all sstables which are already expired.
> >> >
> >> > This line was introduced by
> >> > https://issues.apache.org/jira/browse/CASSANDRA-4022.
> >> >
> >> > Best Regards,
> >> > Cem
> >>
> >>
> >>
> >> --
> >> Yuki Morishita
> >> t:yukim (http://twitter.com/yukim)
> >
> >
>
>
>
> --
> Yuki Morishita
> t:yukim (http://twitter.com/yukim)
>
Re: Cassandra 1.2 TTL histogram problem
Posted by Yuki Morishita <mo...@gmail.com>.
> Can method calculate non-overlapping keys as overlapping?
Yes.
And randomized keys don't matter here since sstables are sorted by
"token" calculated from key by your partitioner, and the method uses
sstable's min/max token to estimate overlap.
On Tue, May 21, 2013 at 4:43 PM, cem <ca...@gmail.com> wrote:
> Thank you very much for the swift answer.
>
> I have one more question about the second part. Can method calculate
> non-overlapping keys as overlapping? I mean it uses max and min tokens and
> column count. They can be very close to each other if random keys are used.
>
> In my use case I generate a GUID for each key and send a single write
> request.
>
> Cem
>
> On Tue, May 21, 2013 at 11:13 PM, Yuki Morishita <mo...@gmail.com> wrote:
>>
>> > Why does Cassandra single table compaction skips the keys that are in
>> > the other sstables?
>>
>> because we don't want to resurrect deleted columns. Say, sstable A has
>> the column with timestamp 1, and sstable B has the same column which
>> deleted at timestamp 2. Then if we purge that column only from sstable
>> B, we would see the column with timestamp 1 again.
>>
>> > I also dont understand why we have this line in worthDroppingTombstones
>> > method
>>
>> What the method is trying to do is to "guess" how many columns that
>> are not in the rows that don't overlap, without actually going through
>> every rows in the sstable. We have statistics like column count
>> histogram, min and max row token for every sstables, we use those in
>> the method to estimate how many columns the two sstables overlap.
>> You may have remainingColumnsRatio of 0 when the two sstables overlap
>> almost entirely.
>>
>>
>> On Tue, May 21, 2013 at 3:43 PM, cem <ca...@gmail.com> wrote:
>> > Hi all,
>> >
>> > I have a question about ticket
>> > https://issues.apache.org/jira/browse/CASSANDRA-3442
>> >
>> > Why does Cassandra single table compaction skips the keys that are in
>> > the
>> > other sstables? Please correct if I am wrong.
>> >
>> > I also dont understand why we have this line in worthDroppingTombstones
>> > method:
>> >
>> > double remainingColumnsRatio = ((double) columns) /
>> > (sstable.getEstimatedColumnCount().count() *
>> > sstable.getEstimatedColumnCount().mean());
>> >
>> > remainingColumnsRatio is always 0 in my case and the droppableRatio is
>> > 0.9. Cassandra skips all sstables which are already expired.
>> >
>> > This line was introduced by
>> > https://issues.apache.org/jira/browse/CASSANDRA-4022.
>> >
>> > Best Regards,
>> > Cem
>>
>>
>>
>> --
>> Yuki Morishita
>> t:yukim (http://twitter.com/yukim)
>
>
--
Yuki Morishita
t:yukim (http://twitter.com/yukim)
Re: Cassandra 1.2 TTL histogram problem
Posted by cem <ca...@gmail.com>.
Thank you very much for the swift answer.
I have one more question about the second part. Can method calculate
non-overlapping keys as overlapping? I mean it uses max and min tokens and
column count. They can be very close to each other if random keys are used.
In my use case I generate a GUID for each key and send a single write
request.
Cem
On Tue, May 21, 2013 at 11:13 PM, Yuki Morishita <mo...@gmail.com> wrote:
> > Why does Cassandra single table compaction skips the keys that are in
> the other sstables?
>
> because we don't want to resurrect deleted columns. Say, sstable A has
> the column with timestamp 1, and sstable B has the same column which
> deleted at timestamp 2. Then if we purge that column only from sstable
> B, we would see the column with timestamp 1 again.
>
> > I also dont understand why we have this line in worthDroppingTombstones
> method
>
> What the method is trying to do is to "guess" how many columns that
> are not in the rows that don't overlap, without actually going through
> every rows in the sstable. We have statistics like column count
> histogram, min and max row token for every sstables, we use those in
> the method to estimate how many columns the two sstables overlap.
> You may have remainingColumnsRatio of 0 when the two sstables overlap
> almost entirely.
>
>
> On Tue, May 21, 2013 at 3:43 PM, cem <ca...@gmail.com> wrote:
> > Hi all,
> >
> > I have a question about ticket
> > https://issues.apache.org/jira/browse/CASSANDRA-3442
> >
> > Why does Cassandra single table compaction skips the keys that are in the
> > other sstables? Please correct if I am wrong.
> >
> > I also dont understand why we have this line in worthDroppingTombstones
> > method:
> >
> > double remainingColumnsRatio = ((double) columns) /
> > (sstable.getEstimatedColumnCount().count() *
> > sstable.getEstimatedColumnCount().mean());
> >
> > remainingColumnsRatio is always 0 in my case and the droppableRatio is
> > 0.9. Cassandra skips all sstables which are already expired.
> >
> > This line was introduced by
> > https://issues.apache.org/jira/browse/CASSANDRA-4022.
> >
> > Best Regards,
> > Cem
>
>
>
> --
> Yuki Morishita
> t:yukim (http://twitter.com/yukim)
>
Re: Cassandra 1.2 TTL histogram problem
Posted by Yuki Morishita <mo...@gmail.com>.
> Why does Cassandra single table compaction skips the keys that are in the other sstables?
because we don't want to resurrect deleted columns. Say, sstable A has
the column with timestamp 1, and sstable B has the same column which
deleted at timestamp 2. Then if we purge that column only from sstable
B, we would see the column with timestamp 1 again.
> I also dont understand why we have this line in worthDroppingTombstones method
What the method is trying to do is to "guess" how many columns that
are not in the rows that don't overlap, without actually going through
every rows in the sstable. We have statistics like column count
histogram, min and max row token for every sstables, we use those in
the method to estimate how many columns the two sstables overlap.
You may have remainingColumnsRatio of 0 when the two sstables overlap
almost entirely.
On Tue, May 21, 2013 at 3:43 PM, cem <ca...@gmail.com> wrote:
> Hi all,
>
> I have a question about ticket
> https://issues.apache.org/jira/browse/CASSANDRA-3442
>
> Why does Cassandra single table compaction skips the keys that are in the
> other sstables? Please correct if I am wrong.
>
> I also dont understand why we have this line in worthDroppingTombstones
> method:
>
> double remainingColumnsRatio = ((double) columns) /
> (sstable.getEstimatedColumnCount().count() *
> sstable.getEstimatedColumnCount().mean());
>
> remainingColumnsRatio is always 0 in my case and the droppableRatio is
> 0.9. Cassandra skips all sstables which are already expired.
>
> This line was introduced by
> https://issues.apache.org/jira/browse/CASSANDRA-4022.
>
> Best Regards,
> Cem
--
Yuki Morishita
t:yukim (http://twitter.com/yukim)