During minor compaction, to reclaim a row tombstone, how cassandra is checking whether the row exists in other sstables? It just checks partition key by bloom filter or checks row key?
For example, there are 3 sstables: s1, s2 and s3. Assume s1 has the row key 'p.c1', where p is partition key and c1 is clustering key. s2 has the row key 'p.c2' and s3 has the tombstone for the row key 'p.c2'. In this case, when minor compaction is triggered on s2 and s3, the row 'p.c2' will be reclaimed after compaction?
Thanks a lot.
Cassandra combines all the fragments of a partition from the active memtable and SSTables to determine if a tombstone can be dropped from an SSTable(s) being compacted.
Similar to read requests, Cassandra checks the memtable, bloom filter, partition key cache or partition summary, and the partition index to locate the fragments of the data/partition/row on disk.
For reference, have a look at How Cassandra reads data. Cheers!
Related
I am new to cassandra and was checking in how cassandra internals work. I checked this article and in this its stated that memtable is stored in sorted order.
But if there's no clustering key or multilple culstering keys, how cassandra store the data in that case in memtable? I want to know what is the criteria of sorting?
There are different ways where data is sorted when it comes to Cassandra.
The term "SSTable" stands for "sorted string table" meaning that the contents of a Cassandra data file are sorted. Data in memtables are sorted by the partition key so that they are already ordered when they are flushed to disk.
Additionally, it also makes it easy for Cassandra to determine whether a partition exists in an SSTable or not since it keeps metadata about the first and last partition key contained in the SSTable.
If the table has clustering columns, the rows are sorted based on the clustering order defined in the table schema. This is the only time where clustering keys are relevant for sorting. Cheers!
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When I request a row with whole primary key, will Cassandra fetch all rows of that partition from SSTable and merge them into memetable, then filter that requested row? or it can find that row with clustering keys and only retrieve one row to memetable?
How does SSTable store data(row by row or column by column, why some SSTables can only contain one column)? if I only request one column, could Cassandra find the location of that particular column and only return that column?
How does Cassandra deal with data that retrieved from SSTable when flush memetable to SSTable, will that data be write to a new SSTable again?
Thanks a lot for any answers.
You should take a look at datastaxacademy.com, specifically the course "DS201: DataStax Enterprise Foundations of Apache Cassandra™". The topics that you are asking for are "Read Path" and "Write path".
I do a select with tracing ON and see:
Skipped 0/1 non-slice-intersecting sstables
included 0 due to tombstones [ReadStage-<N>]
So is it working to ignore tombstones? The trace:
Read 0 live rows and 2 tombstone cells
is clear: it is reading tombstones
Let's say there was a Column A.
You added value x to Column A.
Then you deleted Column A.
Instead of immediately deleting value x, Cassandra will add a marker for Column A which is called tombstone. The tombstone is also an individual record in itself just like the original value x.
Let's say the two updates were written in different sstables (Cassandra storage).
Now when you are reading the value, Cassandra will get the value x and the tombstone for Column A. It will see that tombstone was written after the value x so it will not return any value.
Skipped 0/1 non-slice-intersecting sstables
included 0 due to tombstones
This is basically confirming the same.
Based on talking to some Cassandra admins:
" Skipping sstables is Cassandra telling us it eliminated the tombstones efficiently, this is ok
" Deleting everything in a partition in general helps ensure Cassandra is not bogged down with tombstones
Are rows sorted in Memtable? According to this post:
http://distributeddatastore.blogspot.com/2013/08/cassandra-sstable-storage-format.html
An index will be created when Memtable is flushed into storage (Index.db file). Having a separate index, do rows still need to be sorted in memtable ?
Yes, the writes are stored in the Memtable in sorted order. Check this.
I am new to cassandra,
1) why single column family have 3 sstable.
2) is it each column of the Table ( column family) stored in different nodes in a ring? or completely column family stored in single node ( if I not set replication factor).
example:
Table: message1
SSTable count: 3
Space used (live), bytes: 221521
Space used (total), bytes: 226349
SSTable Compression Ratio: 0.2548965072049006
Number of keys (estimate): 384
Memtable cell count: 7817866
Memtable data size, bytes: 38797312
Memtable switch count: 51
Local read count: 0
Local read latency: 0.000 ms
Local write count: 26539152
Local write latency: 0.000 ms
Pending tasks: 0
Bloom filter false positives: 0
3)
commitlog_sync: periodic
commitlog_sync_period_in_ms: 10000
Commitlog Sync is 10 sec. But the data is not transferred or flushed to disc?.
Memtable data size, bytes: 38797312
SSTable is immutable hence when ever an inserts/updates occur, instead of overwriting the rows, Cassandra writes a new timestamped version of the inserted or updated data in another SSTable. By performing compaction you can merge all the SStable into one single SSTable.
Compaction merges the data in each SSTable data by partition key, selecting the latest data for storage based on its timestamp.
http://www.datastax.com/documentation/cassandra/2.0/cassandra/dml/dml_write_path_c.html
1) why single column family has 3 sstable?
A new SSTABLE is created whenever a memtable is flushed onto the disk. when does this flush happen? When memtable is full or the commit log is full or manual flush is triggered. There is a limit for the number of SSTABLEs in a node, this limit is configurable (min_threshold i.e. maximum number of sstables can be present in a node at a time). When this limit is breached, compaction is triggered which merges SSTABLES and creates a new SSTABLE with the latest timestamped data from all SSTABLES by partition key.
2) is it each column of the Table?
There is no 1-1 mapping b/w column family and sstable. A new SSTABLE is created whenever a memtable is flushed onto disk.
You should look into it.
As per I know Cassandra creates three file for SSTable one for data, other one for bloom filter and third one for indexes.
I think it will help you.