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From Keith Wright <>
Subject key cache hit rate and BF false positive
Date Sat, 27 Jul 2013 03:10:04 GMT
Hi all,

    I am experiencing VERY poor key cache hit rate on my 6 node C* 1.2.4 with Vnode cluster.
 I am using CQL3 with LCS and yesterday increased my SSTable size from default 5 MB to 256
MB although I did not force a major compaction and am instead letting the new size take effect
organically as compactions are normally triggered.  The issue for this email is that my key
cache hit rate is TERRIBLE at < 1% and my bloom filter false positive rate is around 0.2
(see cfstats , info, and create for example table).  I am using the OOTB bloom filter settings
and only 300 Mbs of key cache.  I'm guessing that I need to increase my bloom_filter_fp_chance
from default 0.1 to 0.01 (trigger upgradesstables for it to take effect) and increase my key
cache size (my nodes have 32 GB of RAM and I'm running an 8 GB heap on java 7) but I thought
I would get everyone's opinion first.  My read latency is good but I'm afraid I'll max out
the TPS on my SSD drives which are currently hovering around 40% utilization.

Should I wait to see what effect the sstable size change has first as it will obviously decrease
the number of sstables?
What do you recommend for bloom filter and key cache values?

I tried upping the key cache size to 600 MB but did not see a sizable difference in hit rate.
 I am also planning on trying out row cache but wanted to hold off until I got these values
handled first.  FYI – each node has 3 x 800 GB SSD in JBOD running RF 3.


Column Family: cookie_user_lookup
SSTable count: 13464
SSTables in each level: [1, 3, 101/100, 1022/1000, 10587/10000, 1750]
Space used (live): 106151372078
Space used (total): 106870515200
Number of Keys (estimate): 954198528
Memtable Columns Count: 369605
Memtable Data Size: 170078697
Memtable Switch Count: 668
Read Count: 754323272
Read Latency: 1.029 ms.
Write Count: 130312827
Write Latency: 0.023 ms.
Pending Tasks: 0
Bloom Filter False Positives: 22785986
Bloom Filter False Ratio: 0.24048
Bloom Filter Space Used: 659403424
Compacted row minimum size: 36
Compacted row maximum size: 215
Compacted row mean size: 162

Token            : (invoke with -T/--tokens to see all 256 tokens)
ID               : 6e1a0ac1-6354-40e1-bba4-063c7bb0983d
Gossip active    : true
Thrift active    : true
Load             : 204.64 GB
Generation No    : 1374259760
Uptime (seconds) : 634791
Heap Memory (MB) : 5601.37 / 8072.00
Data Center      : DAL1
Rack             : RAC1
Exceptions       : 78
Key Cache        : size 314572552 (bytes), capacity 314572800 (bytes), 327775491 hits, 3259949221
requests, 0.162 recent hit rate, 14400 save period in seconds
Row Cache        : size 0 (bytes), capacity 0 (bytes), 0 hits, 0 requests, NaN recent hit
rate, 0 save period in seconds

CREATE TABLE cookie_user_lookup (
  cookie text PRIMARY KEY,
  creation_time timestamp,
  opt_out boolean,
  status int,
  user_id timeuuid
  bloom_filter_fp_chance=0.100000 AND
  caching='KEYS_ONLY' AND
  comment='' AND
  dclocal_read_repair_chance=0.000000 AND
  gc_grace_seconds=86400 AND
  read_repair_chance=0.100000 AND
  replicate_on_write='true' AND
  populate_io_cache_on_flush='false' AND
  compaction={'sstable_size_in_mb': '256', 'class': 'LeveledCompactionStrategy'} AND
  compression={'chunk_length_kb': '8', 'crc_check_chance': '0.1', 'sstable_compression': 'LZ4Compressor'};

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