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From "ZAIDI, ASAD" <>
Subject RE: Constant blocking read repair for such a tiny table
Date Wed, 16 Oct 2019 17:56:12 GMT
Wondering if you’ve  disabled  otc_coalescing_strategy  CASSANDRA-12676<>
since you’ve upgraded from 2.x?  also if you found luck by  increasing native_transport_max_threads
 to address blocked NTRs (CASSANDRA-11363)?

From: Patrick Lee []
Sent: Wednesday, October 16, 2019 12:22 PM
Subject: Re: Constant blocking read repair for such a tiny table

haven't really figured this out yet.  it's not a big problem but it is annoying for sure!
the cluster was upgraded from 2.1.16 to 3.11.4.  now my only thing is i'm not sure if had
this type of behavior before the upgrade.  i'm leaning toward a no based on my data but i'm
just not 100% sure.

just 1 table, out of all the ones on the cluster has this behavior. repair has been run few
times via reaper.  even did a nodetool compact on the nodes (since this table is like 1GB
per node..) . just don't see why there would be any inconsistency that would trigger read

any insight you may have would be appreciated!  the real thing that started this digging into
the cluster was during some stress test application team complained about high latency (30ms
at p98).  this cluster is oversized already for this use case with only 14GB of data per node,
there is more than enough ram so all the data is basically cached in ram.  the only thing
that stands out is this crazy read repair.  so this read repair may not be my root issue but
definitely shouldn't be happening like this.

the vm's..
12 cores
82GB ram
1.2TB local ephemeral ssd's

attached the info from 1 of the nodes.

On Tue, Oct 15, 2019 at 2:36 PM Alain RODRIGUEZ <<>>
Hello Patrick,

Still in trouble with this? I must admit I'm really puzzled by your issue. I have no real
idea of what's going on. Would you share with us the output of:

- nodetool status <keyspace>
- nodetool describecluster
- nodetool gossipinfo
- nodetool tpstats

Also you said the app is running for a long time, with no changes. What about Cassandra? Any
recent operations?

I hope that with this information we might be able to understand better and finally be able
to help.

Alain Rodriguez -<>
France / Spain

The Last Pickle - Apache Cassandra Consulting<>

Le ven. 4 oct. 2019 à 00:25, Patrick Lee <<>>
a écrit :
this table was actually leveled compaction before, just changed it to size tiered yesterday
while researching this.

On Thu, Oct 3, 2019 at 4:31 PM Patrick Lee <<>>
its not really time series data.   and it's not updated very often, it would have some updates
but pretty infrequent. this thing should be super fast, on avg it's like 1 to 2ms p99 currently
but if they double - triple the traffic on that table latencies go upward to 20ms to 50ms..
the only odd thing i see is just that there are constant read repairs that follow the same
traffic pattern on the reads, which shows constant writes on the table (from the read repairs),
which after read repair or just normal full repairs (all full through reaper, never ran any
incremental repair) i would expect it to not have any mismatches.  the other 5 tables they
use on the cluster can have the same level traffic all very simple select from table by partition
key which returns a single record

On Thu, Oct 3, 2019 at 4:21 PM John Belliveau <<>>
Hi Patrick,

Is this time series data? If so, I have run into issues with repair on time series data using
the SizeTieredCompactionStrategy. I have had better luck using the TimeWindowCompactionStrategy.


Sent from Mail<>
for Windows 10

From: Patrick Lee<>
Sent: Thursday, October 3, 2019 5:14 PM
Subject: Constant blocking read repair for such a tiny table

I have a cluster that is running 3.11.4 ( was upgraded a while back from 2.1.16 ).  what I
see is a steady rate of read repair which is about 10% constantly on only this 1 table.  Repairs
have been run (actually several times).  The table does not have a lot of writes to it so
after repair, or even after a read repair I would expect it to be fine.  the reason i'm having
to dig into this so much is for the fact that under a much large traffic load than their normal
traffic, latencies are higher than the app team wants

I mean this thing is tiny, it's a 12x12 cluster but this 1 table is like 1GB per node on disk.

the application team is doing reads at LOCAL_QUORUM and I can simulate this on that cluster
by running a query using quorum and/or local_quorum and in the trace can see every time running
the query it comes back with a DigestMismatchException no matter how many times I run it.
that record hasn't been updated by the application for several months.

repairs are scheduled and run every 7 days via reaper, recently in the past week this table
has been repaired at least 3 times.  every time there are mismatches and data streams back
and forth but yet still a constant rate of read repairs.

curious if anyone has any recommendations to look info further or have experienced anything
like this?

this node has been up for 24 hours.. this is the netstats for read repairs
Not sending any streams.
Read Repair Statistics:
Attempted: 7481
Mismatch (Blocking): 11425375
Mismatch (Background): 17
Pool Name                    Active   Pending      Completed   Dropped
Large messages                  n/a         0           1232         0
Small messages                  n/a         0      395903678         0
Gossip messages                 n/a         0         603746         0

example of the schema... some modifications have been made to reduce read_reapair and speculative_retry
while troubleshooting..
CREATE TABLE keyspace.table1 (
    item bigint,
    price int,
    start_date timestamp,
    end_date timestamp,
    created_date timestamp,
    cost decimal,
    list decimal,
    item_id int,
    modified_date timestamp,
    status int,
    PRIMARY KEY ((item, price), start_date, end_date)
) WITH CLUSTERING ORDER BY (start_date ASC, end_date ASC)
    AND read_repair_chance = 0.0
    AND dclocal_read_repair_chance = 0.0
    AND gc_grace_seconds = 864000
    AND bloom_filter_fp_chance = 0.01
    AND caching = { 'keys' : 'ALL', 'rows_per_partition' : 'NONE' }
    AND comment = ''
    AND compaction = { 'class' : 'org.apache.cassandra.db.compaction.SizeTieredCompactionStrategy',
'max_threshold' : 32, 'min_threshold' : 4 }
    AND compression = { 'chunk_length_in_kb' : 4, 'class' : ''
    AND default_time_to_live = 0
    AND speculative_retry = 'NONE'
    AND min_index_interval = 128
    AND max_index_interval = 2048
    AND crc_check_chance = 1.0
    AND cdc = false
    AND memtable_flush_period_in_ms = 0;

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