Hi Jeff,
The cluster was build with 2.1 and has been upgraded to 3.11. It's using
"num_token: 64" and RF=3 for all keyspaces in all DCs.There are over 100 nodes
in each of 6 rings.I don't see any messages of the repair/steaming in
system.log so this shouldn't be related with the repair.I can see messages of
flushing BOTH major app tables AND system.sstable_activity table, but number of
sstables is much higher for the app tables.
Thanks,Jiayong Sun
On Friday, August 13, 2021, 01:43:06 PM PDT, Jeff Jirsa <[email protected]>
wrote:
A very large cluster using vnodes will cause lots of small sstables to stream
in during repair if the cluster is out of sync. This is one of the reasons that
the default number of vnodes was decreased in 4.0. How many nodes in the
cluster, how many DCs, how many vnodes per node, and how many replicas per DC?
You can confirm or eliminate this possibility by checking the origin of the
tiny sstables: are they ACTUALLY flushed from the memtable, or are they
streamed in via repair? Are they all from the sstable-activity table, or are
they the main app table?
On Fri, Aug 13, 2021 at 1:36 PM Bowen Song <[email protected]> wrote:
Hi Jiayong,
That doesn't really match the situation described in the SO question. I
suspected it was related to repairing a table with MV and large partitions, but
based on the information you've given, I was clearly wrong.
A few hundreds MB partitions is not exactly unusual, I don't see that alone
could lead to frequent SSTable flushing. A repair session takes weeks to
complete is a bit worrying in terms of performance and maintainability, but
again it should not cause this issue.
Since we don't know the cause of it, I can see two possible solutions - either
replace the "broken" node, or dig into the logs (remember to turn on the debug
logs) and trying to identify the root cause. I personally would recommend
replacing the problematic node as a quick win.
Cheers,
Bowen
On 13/08/2021 20:31, Jiayong Sun wrote:
Hi Bowen,
We do have reaper repair job scheduled periodically and it can take days even
weeks to complete one round of repair due to large number of rings/nodes.
However, we have paused the repair since we are facing this issue. We do not
use the MV in this cluster. There is major table taking 95% of disk storage and
workload but its Partition Size is around 30 MB. There are a couple small
tables with the Max Partition Size over several hundreds of MB but their total
data size just about a few GB.
Any thoughts?
Thanks, Jiayong
On Friday, August 13, 2021, 03:32:45 AM PDT, Bowen Song <[email protected]>
wrote:
Hi Jiayong,
Sorry I didn't make it clear in my previous email. When I commented on the
RAID0 setup, it was only a comment on the RAID0 setup vs JBOD, and that was not
in relation to the SSTable flushing issue. The part of my previous email after
the "On the frequent SSTable flush issue" line is the part related to the
SSTable flushing issue, and those two questions at the end of it remain valid:
- Did you run repair?
- Do you use materialized views?
and, if I may, I'd also like to add another question:
- Do you have large (> 100 MB) partitions?
Those are the 3 things mentioned in the SO question. I'm trying to find the
connections between the issue you are experiencing and the issue described in
the SO question.
Cheers,
Bowen
On 13/08/2021 01:36, Jiayong Sun wrote:
Hello Bowen,
Thanks for your response. Yes, we are aware of the theory that RAID0 vs
individual JBOD, but all of our clusters are using this RAID0 configuration
through Azure, while only on this cluster we see this issue so it's hardly to
conclude root cause to the disk. This is more like workload related, and we are
seeking feedback here for any other parameters in the yaml that we could tune
for this.
Thanks again, Jiayong Sun
On Thursday, August 12, 2021, 04:55:51 AM PDT, Bowen Song <[email protected]>
wrote:
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 which shouldn't affect 3.x at all
* a StackOverflow question 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 <[email protected]> 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
