Hello Jiayong,
Using multiple disks in a RAID0 for Cassandra data directory is not
recommended. You will get better fault tolerance and often better
performance too with multiple data directories, one on each disk.
If you stick with RAID0, it's not 4 disks, it's 1 from Cassandra's point
of view, because any read or write operation will have to touch all 4
member disks. Therefore, 4 flush writers doesn't make much sense.
On the frequent SSTable flush issue, a quick internet search leads me to:
* an old bug in Cassandra 2.1 - CASSANDRA-8409
<https://issues.apache.org/jira/browse/CASSANDRA-8409> which
shouldn't affect 3.x at all
* a StackOverflow question
<https://stackoverflow.com/questions/61030392/cassandra-node-jvm-hang-during-node-repair-a-table-with-materialized-view>
may be related
Did you run repair? Do you use materialized views?
Regards,
Bowen
On 11/08/2021 15:58, Jiayong Sun wrote:
Hi Erick,
The nodes have 4 SSD (1TB for each but we only use 2.4TB of space.
Current disk usage is about 50%) with RAID0.
Based on number of disks we increased memtable_flush_writers: 4
instead of default of 2.
For the following we set:
- max heap size - 31GB
- memtable_heap_space_in_mb (use default)
- memtable_offheap_space_in_mb (use default)
In the logs, we also noticed system.sstable_activity table has
hundreds of MB or GB of data and constantly flushing:
DEBUG [NativePoolCleaner] <timestamp> ColumnFamilyStore.java:932 -
Enqueuing flush of sstable_activity: 0.293KiB (0%) on-heap, 0.107KiB
(0%) off-heap
DEBUG [NonPeriodicTasks:1] <timestamp> SSTable.java:105 - Deleting
sstable:
/app/cassandra/data/system/sstable_activity-5a1ff267ace03f128563cfae6103c65e/md-103645-big
DEBUG [NativePoolCleaner] <timestamp> ColumnFamilyStore.java:1322 -
Flushing largest CFS(Keyspace='system',
ColumnFamily='sstable_activity') to free up room. Used total:
0.06/1.00, live: 0.00/0.00, flushing: 0.02/0.29, this: 0.00/0.00
Thanks,
Jiayong Sun
On Wednesday, August 11, 2021, 12:06:27 AM PDT, Erick Ramirez
<erick.rami...@datastax.com> wrote:
4 flush writers isn't bad since the default is 2. It doesn't make a
difference if you have fast disks (like NVMe SSDs) because only 1
thread gets used.
But if flushes are slow, the work gets distributed to 4 flush writers
so you end up with smaller flush sizes although it's difficult to tell
how tiny the SSTables would be without analysing the logs and overall
performance of your cluster.
Was there a specific reason you decided to bump it up to 4? I'm just
trying to get a sense of why you did it since it might provide some
clues. Out of curiosity, what do you have set for the following?
- max heap size
- memtable_heap_space_in_mb
- memtable_offheap_space_in_mb
