Database Reference
In-Depth Information
data is persisted) to be done only after the flush is done or the batch window is over,
whichever is sooner. This means the batch modes will always be slower than the periodic
modes.
For most practical cases, the default periodic value and default
fsync()
period of ten
seconds will do just fine.
column_index_size_in_kb
The
column_index_size_in kb
property tells Cassandra to add a column index if
the size of a row (serialized size) grows beyond the KBs mentioned by this property. In
other words, if row size is 314 KB and
column_index_size_in_kb
is set to 64
(KB), there will be a column index with at least five entries, each containing the start and
the finish column name in the chunk and its offset and width.
If the row contains many columns (wide rows) or you have columns with really large-
sized values, you may want to increase the default. It has a con; for a large column index
in KB, Cassandra will need to read at least this much amount of data, even a single
column of a small row with small values needs to be read. On the other hand, a small
value for this property, large index data will need to be read at each access. The default is
okay for most of the cases.
commitlog_total_space_in_mb
The commit log file is
memory mapped
(
mmap
). This means that the file takes the virtu-
al address space. Cassandra flushes any unflushed MemTables that exist in the oldest
memory mapped commit log segment to the disk. Thinking from the I/O point of view, it
does not make sense to keep this property small because the total space of smaller commit
logs will be filled up quickly, requiring frequent write to the disk and higher disk I/O. On
the other hand, we do not want commit logs to hog all the memory. Note that the data that
was not flushed to the disk. In the event of a shutdown, it is replayed from the commit log.
So, the larger the commit log, the more the replay time it will take to restart.
The default for the 32-bit JVM is 32 MB and for 64-bit JVM is 1024 MB. You may tune it
based on the memory availability on the node.
