Thanks. So in your use case, you actually keep parts of the same series in different rows, to keep the rows from getting too wide? I thought Cassandra worked OK with millions of columns per row. If I don't have to split a row into parts, that keep the data model simpler for me. (Otherwise, if I want to split row and reassemble in client code, I could just use RDBMS :-)


On Tue, Feb 12, 2013 at 12:07 PM, Hiller, Dean <Dean.Hiller@nrel.gov> wrote:
We are using cassandra for time series as well with PlayOrm.  A guess is
we will be doing equal reads and writes on all the data going back 10
years(currently in production we are write heavy right now).  We have
60,000 virtual tables (one table per sensor we read from and yes we have
that many sensors).  We partition with PlayOrm partitioning one months
worth for each of the virtual tables.  This gives us a wide row index into
each partition that playorm creates and the rest of the data varies
between very narrow tables (one column) and tables with around 20 columns.
 It seems to be working extremely well so far and we run it on 6 cassandra
nodes as well.

Anyways, thought I would share as perhaps it helps you understand your use
case.

Later,
Dean

On 2/12/13 8:08 AM, "Edward Capriolo" <edlinuxguru@gmail.com> wrote:

>Your use case is 100% on the money for Cassandra. But let me take a
>chance to slam the other NoSQLs. (not really slam but you know)
>
>Riak is a key-value store. It is not a column family store where a
>rowkey has a map of sorted values. This makes the time series more
>awkward as the time series has to span many rows, rather then one
>large row.
>
>HBase has similiar problems with time-series. On one hand if your
>rowkeys are series you get hotspots, if you columns are time series
>you run into two subtle issues. Last I check hbase's on disk format
>repeats the key each time (somewhat wasteful)
>
>key,column,value
>key,column,value
>key,column,value
>
>Also there are issues with really big rows, although they are dealt
>with in a similiar way to really wide rows in cassandra, just use time
>as part of the row key and the rows will not get that large.
>
>I do not think you need leveled compaction for an append only
>workload, although it might be helpful depending on how long you want
>to keep these rows. If you are not keeping them very long possibly
>leveled would keep the on disk size smaller.
>
>Column TTLs in cassandra do not require extra storage. It is a very
>efficient way to do this. Otherwise you have to scan through your data
>with some offline process and delete.
>
>Do not worry about gc_grace to much. The moral is because of
>distributed deletes some data lives on disk for a while after it is
>deleted. All this means is you need "some" more storage then just the
>space for your live data.
>
>Don't use row cache with wide rows REPEAT Don't use row cache with wide
>rows
>
>Compaction throughput is metered on each node (again not a setting to
>worry about
>
>if you are hitting flush_largest_memtables_at and
>reduce_cache_capacity_to it basically means your have over tuned or
>you do not have enough hardware. These are mostly emergency valves and
>if you are setup well these are not a factor. They are only around to
>relieve memory pressure to prevent the node from hitting a cycle where
>it is in GC more then it is in serving mode.
>
>Whew!
>
>Anyway. Nice to see that you are trying to understand the knobs,
>before kicking the tires.
>
>On Tue, Feb 12, 2013 at 5:55 AM, Boris Solovyov
><boris.solovyov@gmail.com> wrote:
>> Hello list!
>>
>> I have application with following characteristics:
>>
>> data is time series, tens of millions of series at 1-sec granularity,
>>like
>> stock ticker data
>> values are timestamp, integer (uint64)
>> data is append only, never update
>> data don't write in distant past, maybe sometimes write 10 sec ago but
>>not
>> more
>> data is write mostly, like 99.9% write I think
>> most read will be of recent data, always in range of timestamps
>> data needs purge after some time, ex. 1 week
>>
>> I consider to use Cassandra. No other existing database (HBase, Riak,
>>etc)
>> seems well suited for this.
>>
>> Questions:
>>
>> Did I miss some others database that could work? Please suggest me if
>>you
>> know one.
>> What are benefits or drawbacks of leveled compaction for this workload?
>> Setting column TTL seems bad choice due to extra storage. Agree? Is
>> efficient to run routine batch job to purge oldest data? Is there will
>>be
>> any gotcha with that (like fullscan of something instead of just oldest,
>> maybe?)
>> Will column index beneficial? If reads are scans, does it matter, or is
>>it
>> just extra work and storage space to maintain, without much benefit
>> especially since reads are rare?
>> How gc_grace_seconds impacts operations in this workload? Will purges
>>of old
>> data leave sstables mostly obsolete, rather than sparsely obsolete? I
>>think
>> they will. So, after purge, tombstones can be GC shortly, no need for
>> default 10 days grace period. BUT, I read in docs that if
>>gc_grace_seconds
>> is short, then nodetool repair needs run quite often. Is that true? Why
>> would that be needed in my use case?
>> Related question: is it sensible to set tombstone_threshold to 1.0 but
>> tombstone_compaction_interval to something short, like 1 hour? I suppose
>> this depends on whether I am correct that SSTables will be deleted
>>entirely,
>> instead of just getting sparse.
>> Should I disable row_cache_provider? It invalidates every row on update,
>> right? I will be updating rows constantly, so it seems not benefitial.
>> Docs say "compaction_throughput_mb_per_sec" is per "entire system." Does
>> that mean per NODE, or per ENTIRE CLUSTER? Will this cause trouble with
>> periodic deletions of expired columns? Do I need to make sure my purges
>>of
>> old data are trickled out over time to avoid huge overhead of
>>compaction?
>> But in that case, SSTables will become sparsely deleted, right? And then
>> re-compacted, which seems wasteful if the remaining data will soon be
>>purged
>> again and there will be another re-compaction. So this is partially why
>>I
>> asked about tombstone-threshold and compaction interval -- I think is
>>best
>> if I can purge data in such a way that Cassandra never recompacts
>>SsTables,
>> but just realizes "oh, whole thing is dead, I can delete, no work
>>needed."
>> But I am not sure if my considered settings will have unintended
>> consequence.
>> Finally, with proposed workload, will there be troubles with
>> flush_larges_memtables_at and reduce_cache_capacity_to,
>> reduce_cache_sizes_at? These are describe as "emergency measures" in
>>docs.
>> If my workload is edge case that could trigger bad emergency-measure
>> behavior I hope you can say me that :-)
>>
>> Many thanks!
>>
>> Boris