Enable or disable sstable compression?

In cassandra 2.0.6, there are a few compression for sstables, the default is LZ4Compressor. There are others such as DeflateCompressor, SnappyCompressor or
do not compress the sstables at all.

You can read more about compression at official documentation as found it here.

With this blog, I will create two scenarios where first scenario is with enable compression and another scenario is without compression. This is the only different for both scenarios.

So I have create 50 thousands insert statement with cql and then insert using by feeding to cqlsh. So first , the schema below with LZ4Compressor compression and leave value for key sstable_compression empty for no compression.

CREATE TABLE users (
  user_id text,
  age int,
  first text,
  last text,
  middle text,
  PRIMARY KEY (user_id)
) WITH
  bloom_filter_fp_chance=0.010000 AND
  caching='KEYS_ONLY' AND
  comment='storing user data' AND
  dclocal_read_repair_chance=0.000000 AND
  gc_grace_seconds=864000 AND
  index_interval=128 AND
  read_repair_chance=0.100000 AND
  replicate_on_write='true' AND
  populate_io_cache_on_flush='false' AND
  default_time_to_live=0 AND
  speculative_retry='99.0PERCENTILE' AND
  memtable_flush_period_in_ms=0 AND
  compaction={'class': 'SizeTieredCompactionStrategy'} AND
  compression={'sstable_compression': 'LZ4Compressor'};

CREATE INDEX idxAge ON users (age);

CREATE INDEX idxLast ON users (last);

jason@localhost:~$ wc -l data.cql
50000 data.cql
jason@localhost:~$ cqlsh 192.168.0.2 9160 -k jw_schema1 -f data.cql
jason@localhost:~$

so looks good, that we have total rows of 50 thousands.

cqlsh:jw_schema1> select count(*) from users limit 100000;

 count
-------
 50000

(1 rows)

cqlsh:jw_schema1>

Ran nodetool repair, flush, cleanup and then compact. With compression enable, the sstable count only 1 and the total filesize in this directory is about 4.5MB.

jason@localhost:/var/lib/cassandra/data/jw_schema1/users$ ls -l
total 4576
-rw-r--r-- 1 cassandra cassandra     179 Apr 15 21:02 jw_schema1-users.idxAge-jb-1-CompressionInfo.db
-rw-r--r-- 1 cassandra cassandra  599421 Apr 15 21:02 jw_schema1-users.idxAge-jb-1-Data.db
-rw-r--r-- 1 cassandra cassandra     136 Apr 15 21:02 jw_schema1-users.idxAge-jb-1-Filter.db
-rw-r--r-- 1 cassandra cassandra    1800 Apr 15 21:02 jw_schema1-users.idxAge-jb-1-Index.db
-rw-r--r-- 1 cassandra cassandra    4392 Apr 15 21:02 jw_schema1-users.idxAge-jb-1-Statistics.db
-rw-r--r-- 1 cassandra cassandra      68 Apr 15 21:02 jw_schema1-users.idxAge-jb-1-Summary.db
-rw-r--r-- 1 cassandra cassandra      79 Apr 15 21:02 jw_schema1-users.idxAge-jb-1-TOC.txt
-rw-r--r-- 1 cassandra cassandra     179 Apr 15 21:02 jw_schema1-users.idxLast-jb-1-CompressionInfo.db
-rw-r--r-- 1 cassandra cassandra  598579 Apr 15 21:02 jw_schema1-users.idxLast-jb-1-Data.db
-rw-r--r-- 1 cassandra cassandra      16 Apr 15 21:02 jw_schema1-users.idxLast-jb-1-Filter.db
-rw-r--r-- 1 cassandra cassandra     680 Apr 15 21:02 jw_schema1-users.idxLast-jb-1-Index.db
-rw-r--r-- 1 cassandra cassandra    4392 Apr 15 21:02 jw_schema1-users.idxLast-jb-1-Statistics.db
-rw-r--r-- 1 cassandra cassandra      71 Apr 15 21:02 jw_schema1-users.idxLast-jb-1-Summary.db
-rw-r--r-- 1 cassandra cassandra      79 Apr 15 21:02 jw_schema1-users.idxLast-jb-1-TOC.txt
-rw-r--r-- 1 cassandra cassandra     971 Apr 15 21:02 jw_schema1-users-jb-1-CompressionInfo.db
-rw-r--r-- 1 cassandra cassandra 2387391 Apr 15 21:02 jw_schema1-users-jb-1-Data.db
-rw-r--r-- 1 cassandra cassandra   62512 Apr 15 21:02 jw_schema1-users-jb-1-Filter.db
-rw-r--r-- 1 cassandra cassandra  938894 Apr 15 21:02 jw_schema1-users-jb-1-Index.db
-rw-r--r-- 1 cassandra cassandra    4391 Apr 15 21:02 jw_schema1-users-jb-1-Statistics.db
-rw-r--r-- 1 cassandra cassandra    6615 Apr 15 21:02 jw_schema1-users-jb-1-Summary.db
-rw-r--r-- 1 cassandra cassandra      79 Apr 15 21:02 jw_schema1-users-jb-1-TOC.txt
drwxr-xr-x 2 cassandra cassandra    4096 Apr 15 20:57 snapshots
jason@localhost:/var/lib/cassandra/data/jw_schema1/users$

Right now without compression, the total file size is about 11MB. Noticed that, the size is almost double and the sstable count is two.

jason@localhost:/var/lib/cassandra/data/jw_schema1/users$ ls -l
total 10860
-rw-r--r-- 1 cassandra cassandra      48 Apr 15 21:23 jw_schema1-users.idxAge-jb-1-CRC.db
-rw-r--r-- 1 cassandra cassandra  687656 Apr 15 21:23 jw_schema1-users.idxAge-jb-1-Data.db
-rw-r--r-- 1 cassandra cassandra      78 Apr 15 21:23 jw_schema1-users.idxAge-jb-1-Digest.sha1
-rw-r--r-- 1 cassandra cassandra     136 Apr 15 21:23 jw_schema1-users.idxAge-jb-1-Filter.db
-rw-r--r-- 1 cassandra cassandra    1800 Apr 15 21:23 jw_schema1-users.idxAge-jb-1-Index.db
-rw-r--r-- 1 cassandra cassandra    4392 Apr 15 21:23 jw_schema1-users.idxAge-jb-1-Statistics.db
-rw-r--r-- 1 cassandra cassandra      68 Apr 15 21:23 jw_schema1-users.idxAge-jb-1-Summary.db
-rw-r--r-- 1 cassandra cassandra      79 Apr 15 21:23 jw_schema1-users.idxAge-jb-1-TOC.txt
-rw-r--r-- 1 cassandra cassandra      32 Apr 15 21:24 jw_schema1-users.idxAge-jb-2-CRC.db
-rw-r--r-- 1 cassandra cassandra  455238 Apr 15 21:24 jw_schema1-users.idxAge-jb-2-Data.db
-rw-r--r-- 1 cassandra cassandra      78 Apr 15 21:24 jw_schema1-users.idxAge-jb-2-Digest.sha1
-rw-r--r-- 1 cassandra cassandra     136 Apr 15 21:24 jw_schema1-users.idxAge-jb-2-Filter.db
-rw-r--r-- 1 cassandra cassandra    1800 Apr 15 21:24 jw_schema1-users.idxAge-jb-2-Index.db
-rw-r--r-- 1 cassandra cassandra    4393 Apr 15 21:24 jw_schema1-users.idxAge-jb-2-Statistics.db
-rw-r--r-- 1 cassandra cassandra      68 Apr 15 21:24 jw_schema1-users.idxAge-jb-2-Summary.db
-rw-r--r-- 1 cassandra cassandra      79 Apr 15 21:24 jw_schema1-users.idxAge-jb-2-TOC.txt
-rw-r--r-- 1 cassandra cassandra      48 Apr 15 21:23 jw_schema1-users.idxLast-jb-1-CRC.db
-rw-r--r-- 1 cassandra cassandra  685677 Apr 15 21:23 jw_schema1-users.idxLast-jb-1-Data.db
-rw-r--r-- 1 cassandra cassandra      79 Apr 15 21:23 jw_schema1-users.idxLast-jb-1-Digest.sha1
-rw-r--r-- 1 cassandra cassandra      16 Apr 15 21:23 jw_schema1-users.idxLast-jb-1-Filter.db
-rw-r--r-- 1 cassandra cassandra     425 Apr 15 21:23 jw_schema1-users.idxLast-jb-1-Index.db
-rw-r--r-- 1 cassandra cassandra    4392 Apr 15 21:23 jw_schema1-users.idxLast-jb-1-Statistics.db
-rw-r--r-- 1 cassandra cassandra      71 Apr 15 21:23 jw_schema1-users.idxLast-jb-1-Summary.db
-rw-r--r-- 1 cassandra cassandra      79 Apr 15 21:23 jw_schema1-users.idxLast-jb-1-TOC.txt
-rw-r--r-- 1 cassandra cassandra      32 Apr 15 21:24 jw_schema1-users.idxLast-jb-2-CRC.db
-rw-r--r-- 1 cassandra cassandra  453259 Apr 15 21:24 jw_schema1-users.idxLast-jb-2-Data.db
-rw-r--r-- 1 cassandra cassandra      79 Apr 15 21:24 jw_schema1-users.idxLast-jb-2-Digest.sha1
-rw-r--r-- 1 cassandra cassandra      16 Apr 15 21:24 jw_schema1-users.idxLast-jb-2-Filter.db
-rw-r--r-- 1 cassandra cassandra     287 Apr 15 21:24 jw_schema1-users.idxLast-jb-2-Index.db
-rw-r--r-- 1 cassandra cassandra    4393 Apr 15 21:24 jw_schema1-users.idxLast-jb-2-Statistics.db
-rw-r--r-- 1 cassandra cassandra      71 Apr 15 21:24 jw_schema1-users.idxLast-jb-2-Summary.db
-rw-r--r-- 1 cassandra cassandra      79 Apr 15 21:24 jw_schema1-users.idxLast-jb-2-TOC.txt
-rw-r--r-- 1 cassandra cassandra     288 Apr 15 21:23 jw_schema1-users-jb-1-CRC.db
-rw-r--r-- 1 cassandra cassandra 4612770 Apr 15 21:23 jw_schema1-users-jb-1-Data.db
-rw-r--r-- 1 cassandra cassandra      71 Apr 15 21:23 jw_schema1-users-jb-1-Digest.sha1
-rw-r--r-- 1 cassandra cassandra   37880 Apr 15 21:23 jw_schema1-users-jb-1-Filter.db
-rw-r--r-- 1 cassandra cassandra  564480 Apr 15 21:23 jw_schema1-users-jb-1-Index.db
-rw-r--r-- 1 cassandra cassandra    4391 Apr 15 21:23 jw_schema1-users-jb-1-Statistics.db
-rw-r--r-- 1 cassandra cassandra    3984 Apr 15 21:23 jw_schema1-users-jb-1-Summary.db
-rw-r--r-- 1 cassandra cassandra      79 Apr 15 21:23 jw_schema1-users-jb-1-TOC.txt
-rw-r--r-- 1 cassandra cassandra     192 Apr 15 21:24 jw_schema1-users-jb-2-CRC.db
-rw-r--r-- 1 cassandra cassandra 3015018 Apr 15 21:24 jw_schema1-users-jb-2-Data.db
-rw-r--r-- 1 cassandra cassandra      71 Apr 15 21:24 jw_schema1-users-jb-2-Digest.sha1
-rw-r--r-- 1 cassandra cassandra   24648 Apr 15 21:24 jw_schema1-users-jb-2-Filter.db
-rw-r--r-- 1 cassandra cassandra  374414 Apr 15 21:24 jw_schema1-users-jb-2-Index.db
-rw-r--r-- 1 cassandra cassandra    4391 Apr 15 21:24 jw_schema1-users-jb-2-Statistics.db
-rw-r--r-- 1 cassandra cassandra    2672 Apr 15 21:24 jw_schema1-users-jb-2-Summary.db
-rw-r--r-- 1 cassandra cassandra      79 Apr 15 21:24 jw_schema1-users-jb-2-TOC.txt

With current hardware setup, which is loaded, with sstable compression enable, at times, the request get rpc timeout but at times, the result is returned. However without compression on sstable, all the requests executed get timeout. Below are the query perform via cqlsh.

cqlsh:jw_schema1> select * from users where age > 95 and last = 'smith' allow filtering;
Request did not complete within rpc_timeout.

Apparently enable compression does improve reading speed and saving disk size.

Research into cassandra nodetool cfhistograms and interpret statistics

What is nodetool cfhistogram?

According to the official documentation definition: The nodetool cfhistograms command provides statistics about a table, including read/write latency, row size, column count, and number of SSTables.

If you noticed the picture output below, it is entirely different than the cfhistogram output in cassandra 2.0.6 . Apparently output of cfhistograms is simplified and improved! You can find more information about this improvement here. To get the existing way of output, give −−compact to the nodetool as a parameter.



Okay, let's start by issue command nodetool cfhistograms to our cluster.

jason@localhost:~$ nodetool -h localhost cfhistograms jw_schema1 users
jw_schema1/users histograms

SSTables per Read
1 sstables: 997

Write Latency (microseconds)
No Data

Read Latency (microseconds)
  103 us: 1
  124 us: 15
  149 us: 28
  179 us: 131
  215 us: 306
  258 us: 373
  310 us: 66
  372 us: 17
  446 us: 6
  535 us: 21
  642 us: 10
  770 us: 2
  924 us: 1
 1109 us: 3
 1331 us: 1
 1597 us: 1
 1916 us: 3
 2299 us: 0
 2759 us: 2
 3311 us: 1
 3973 us: 0
 4768 us: 0
 5722 us: 1
 6866 us: 0
 8239 us: 1
 9887 us: 4
11864 us: 1
14237 us: 1
17084 us: 1

Partition Size (bytes)
149 bytes: 3

Cell Count per Partition
5 cells: 3

The statistics is a bit difficult to understand if you do not know what does it mean. Let's begin by studying into the cfhistograms codes.

private void printCfHistograms(String keySpace, String columnFamily, PrintStream output, boolean compactFormat)
{
   ColumnFamilyStoreMBean store = this.probe.getCfsProxy(keySpace, columnFamily);

   // default is 90 offsets
   long[] offsets = new EstimatedHistogram().getBucketOffsets();

   long[] rrlh = store.getRecentReadLatencyHistogramMicros();
   long[] rwlh = store.getRecentWriteLatencyHistogramMicros();
   long[] sprh = store.getRecentSSTablesPerReadHistogram();
   long[] ersh = store.getEstimatedRowSizeHistogram();
   long[] ecch = store.getEstimatedColumnCountHistogram();

   output.println(String.format("%s/%s histograms", keySpace, columnFamily));
   output.println("");

   if (compactFormat)
   {
      output.println(String.format("%-10s%10s%18s%18s%18s%18s",
               "Offset", "SSTables", "Write Latency", "Read Latency", "Partition Size", "Cell Count"));
      output.println(String.format("%-10s%10s%18s%18s%18s%18s",
               "", "", "(micros)", "(micros)", "(bytes)", ""));

      for (int i = 0; i < offsets.length; i++)
      {
         output.println(String.format("%-10d%10s%18s%18s%18s%18s",
            offsets[i],
               (i < sprh.length ? sprh[i] : "0"),
               (i < rwlh.length ? rwlh[i] : "0"),
               (i < rrlh.length ? rrlh[i] : "0"),
               (i < ersh.length ? ersh[i] : "0"),
               (i < ecch.length ? ecch[i] : "0")));
      }
   }
   else
   {
       output.println("SSTables per Read");
       printHistogram(sprh, offsets, "sstables", output);

       output.println("Write Latency (microseconds)");
       printHistogram(rwlh, offsets, "us", output);

       output.println("Read Latency (microseconds)");
       printHistogram(rrlh, offsets, "us", output);

       output.println("Partition Size (bytes)");
       printHistogram(ersh, offsets, "bytes", output);

       output.println("Cell Count per Partition");
       printHistogram(ecch, offsets, "cells", output);
   }
}

Essentially a proxy ColumnFamilyStoreMBean is made through jmx ($ jconsole service:jmx:rmi:///jndi/rmi://192.168.0.2:7199/jmxrmi also see picture below) based on the previous keyspace and column family specified in the nodetool parameter. The default bucket offset will always be 90. Thus if you carefully analyzed the row output of the compact statistics, you will noticed exactly 90 rows each time nodetool cfhistogram command is triggered.



You would ask, why would 90 bucket offsets? Well according to the codes documentation:

The series of values to which the counts in `buckets` correspond:
1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 17, 20, etc.
Thus, a `buckets` of [0, 0, 1, 10] would mean we had seen one value of 3 and 10 values of 4.

The series starts at 1 and grows by 1.2 each time (rounding and removing duplicates). It goes from 1
to around 36M by default (creating 90+1 buckets), which will give us timing resolution from microseconds to
36 seconds, with less precision as the numbers get larger.

Each bucket represents values from (previous bucket offset, current offset].

Depending if parameter compact is specified, the output will be different. There are six metrics exposed. We will take a closer look.

• offset | the bucket offset

Bucket offset from 149 (exclusive) to 179 (inclusive). Essentially this bucket offset contain latency from 149 microseconds until 179 microseconds.

• SSTables | recent SSTables per read

With each read, total of sstables accessed accountable for. Note that for each nodetool cfhistograms trigger for this keyspace and column family, this metric will be reset.

This metric will increase if there is any call to CollationController.java or CacheService.java

• Write Latency (micros) | recent write latency histogram in microseconds.

An array representing the latency histogram for write in microseconds. Note that for each nodetool cfhistograms trigger for this keyspace and column family, this metric will be reset.

This metric will increase if there is any call to ColumnFamilyStore.java, StorageProxy.java or WeightedQueue.java .

• Read Latency (micros) | recent read latency histogram in Microseconds.

An array representing the latency histogram for read in microseconds. Note that for each nodetool cfhistograms trigger for this keyspace and column family, this metric will be reset.

• Partition Size (bytes ) | estimated row size histogram

As estimation of row size in bytes. Note that for each nodetool cfhistograms trigger for this keyspace and column family, this metric will NOT reset.

The metric is collected by iterating over the sstables, and get the estimated row size in bytes.

• Cell Count | estimated column count histogram

Estimated number of columns. Note that for each nodetool cfhistograms trigger for this keyspace and column family, this metric will NOT reset.

The metric is collected by iterating over the sstables, and get the estimated column count.

So with these interpretation from the codes, let's take another compact form cfhistogram to interpret the metrics. First, we will make start by make some statistics:

cqlsh:jw_schema1> select * from users where age > 5 and age < 50 and last = 'smith' allow filtering;

jason@localhost:~$ nodetool -h localhost cfhistograms jw_schema1 users  -c
jw_schema1/users histograms

Offset      SSTables     Write Latency      Read Latency    Partition Size        Cell Count
                              (micros)          (micros)           (bytes)
1                997                 0                 0                 0                 0
2                  0                 0                 0                 0                 0
3                  0                 0                 0                 0                 0
4                  0                 0                 0                 0                 0
5                  0                 0                 0                 0              1000
6                  0                 0                 0                 0                 0
7                  0                 0                 0                 0                 0
8                  0                 0                 0                 0                 0
10                 0                 0                 0                 0                 0
12                 0                 0                 0                 0                 0
14                 0                 0                 0                 0                 0
17                 0                 0                 0                 0                 0
20                 0                 0                 0                 0                 0
24                 0                 0                 0                 0                 0
29                 0                 0                 0                 0                 0
35                 0                 0                 0                 0                 0
42                 0                 0                 0                 0                 0
50                 0                 0                 0                 0                 0
60                 0                 0                 0                 0                 0
72                 0                 0                 0                 0                 0
86                 0                 0                 0                 0                 0
103                0                 0                 0                 0                 0
124                0                 0                 0                 0                 0
149                0                 0                 0               999                 0
179                0                 0                 0                 1                 0
215                0                 0                 0                 0                 0
258                0                 0                 0                 0                 0
310                0                 0                 0                 0                 0
372                0                 0                 0                 0                 0
446                0                 0                 0                 0                 0
535                0                 0                 0                 0                 0
642                0                 0                 0                 0                 0
770                0                 0                 0                 0                 0
924                0                 0                 0                 0                 0
1109               0                 0                 0                 0                 0
1331               0                 0                51                 0                 0
1597               0                 0               491                 0                 0
1916               0                 0                95                 0                 0
2299               0                 0                53                 0                 0
2759               0                 0                84                 0                 0
3311               0                 0                95                 0                 0
3973               0                 0                41                 0                 0
4768               0                 0                32                 0                 0
5722               0                 0                25                 0                 0
6866               0                 0                 9                 0                 0
8239               0                 0                 7                 0                 0
9887               0                 0                 6                 0                 0
11864              0                 0                 4                 0                 0
14237              0                 0                 0                 0                 0
17084              0                 0                 2                 0                 0
20501              0                 0                 0                 0                 0
24601              0                 0                 0                 0                 0
29521              0                 0                 0                 0                 0
35425              0                 0                 0                 0                 0
42510              0                 0                 1                 0                 0
51012              0                 0                 0                 0                 0
61214              0                 0                 0                 0                 0
73457              0                 0                 0                 0                 0
88148              0                 0                 0                 0                 0
105778             0                 0                 1                 0                 0
126934             0                 0                 0                 0                 0
152321             0                 0                 0                 0                 0
182785             0                 0                 0                 0                 0
219342             0                 0                 0                 0                 0
263210             0                 0                 0                 0                 0
315852             0                 0                 0                 0                 0
379022             0                 0                 0                 0                 0
454826             0                 0                 0                 0                 0
545791             0                 0                 0                 0                 0
654949             0                 0                 0                 0                 0
785939             0                 0                 0                 0                 0
943127             0                 0                 0                 0                 0
1131752            0                 0                 0                 0                 0
1358102            0                 0                 0                 0                 0
1629722            0                 0                 0                 0                 0
1955666            0                 0                 0                 0                 0
2346799            0                 0                 0                 0                 0
2816159            0                 0                 0                 0                 0
3379391            0                 0                 0                 0                 0
4055269            0                 0                 0                 0                 0
4866323            0                 0                 0                 0                 0
5839588            0                 0                 0                 0                 0
7007506            0                 0                 0                 0                 0
8409007            0                 0                 0                 0                 0
10090808           0                 0                 0                 0                 0
12108970           0                 0                 0                 0                 0
14530764           0                 0                 0                 0                 0
17436917           0                 0                 0                 0                 0
20924300           0                 0                 0                 0                 0
25109160           0                 0                 0                 0                 0

• There are 51 read requests spend time from 1109 microsecond to 1331 microsecond.


• 997 sstables were read and spent time 1 microsecond.


• Because this is a read operation, (cql select statement), there is no write latency involved.


• The mean size for 999 partition is 149 bytes and another one is 179 bytes.


• There are 1000 partition with 5 cells.

These metric is good for monitoring if you can poll periodically and plot them into graphs. Note that, those methods covered above, many had been deprecated in this cassandra version and probably in the coming cassandra, it will be removed and that they will have better way of depicting the metric. If you started on older cassandra version for example, pre-cassandra 1.1, the cell is correspond to column whilst partition is correspond to row.

Thank you.

Learn and play with cassandra 2.0.6 snapshot and restore

Snapshot of cassandra appearing as early as cassandra version 0.4.0 beta. Today, we are going to learn on cassandra snapshot. Note that if you run snapshot on a node in a cluster, it only snapshot on that node. If you want to snapshot for all nodes in a cluster,

it is much more efficient to use a parallel ssh such as clusterssh or pssh.

Fundamentally, when snapshot is executed, it copy the sstables into a snapshot directory. So be notice that if you have a huge node load, it require two times the disk space of that server and it may spike the I/O activity on that node too if large amount of sstables is being snapshot.

Let's get down to the work.

First, ensure at least the table (column family) has data.

cqlsh:jw_schema1> select * from users;

 user_id | age | first | last     | middle
---------+-----+-------+----------+--------
       3 |  34 |  john |    smith |     a
       2 |  35 |  olee |    smith |     b
       1 |  33 |   dan |    bar   |     c

(3 rows)

Then take a snapshot, for instance, I only take a snapshot of this keyspace, jw_schema1 and table users. What that does is that, cassandra will flush the data to sstable before snapshot is taken. For option such as giving a meaningful snapshot a name, check out command nodetool help.

jason@localhost:~$ nodetool -h localhost snapshot jw_schema1 -cf users
Requested creating snapshot for: jw_schema1 and table: users
Snapshot directory: 1397292720524

The snapshot made will be stored at <data_file_directories> that you set in cassandra.yaml file. So for instance,

jason@localhost:/var/lib/cassandra/data/jw_schema1/users$ ls -l snapshots/1397292720524/
total 96K
-rw-r--r-- 2 cassandra cassandra   16 Apr 12 16:52 jw_schema1-users.idxAge-jb-1-Filter.db
-rw-r--r-- 2 cassandra cassandra   54 Apr 12 16:52 jw_schema1-users.idxAge-jb-1-Index.db
-rw-r--r-- 2 cassandra cassandra   76 Apr 12 16:52 jw_schema1-users.idxAge-jb-1-Data.db
-rw-r--r-- 2 cassandra cassandra   43 Apr 12 16:52 jw_schema1-users.idxAge-jb-1-CompressionInfo.db
-rw-r--r-- 2 cassandra cassandra 4.3K Apr 12 16:52 jw_schema1-users.idxAge-jb-1-Statistics.db
-rw-r--r-- 2 cassandra cassandra   79 Apr 12 16:52 jw_schema1-users.idxAge-jb-1-TOC.txt
-rw-r--r-- 2 cassandra cassandra   68 Apr 12 16:52 jw_schema1-users.idxAge-jb-1-Summary.db
-rw-r--r-- 2 cassandra cassandra   16 Apr 12 16:52 jw_schema1-users.idxLast-jb-1-Filter.db
-rw-r--r-- 2 cassandra cassandra   58 Apr 12 16:52 jw_schema1-users.idxLast-jb-1-Index.db
-rw-r--r-- 2 cassandra cassandra   87 Apr 12 16:52 jw_schema1-users.idxLast-jb-1-Data.db
-rw-r--r-- 2 cassandra cassandra   43 Apr 12 16:52 jw_schema1-users.idxLast-jb-1-CompressionInfo.db
-rw-r--r-- 2 cassandra cassandra 4.3K Apr 12 16:52 jw_schema1-users.idxLast-jb-1-Statistics.db
-rw-r--r-- 2 cassandra cassandra   79 Apr 12 16:52 jw_schema1-users.idxLast-jb-1-TOC.txt
-rw-r--r-- 2 cassandra cassandra   75 Apr 12 16:52 jw_schema1-users.idxLast-jb-1-Summary.db
-rw-r--r-- 2 cassandra cassandra   16 Apr 12 16:52 jw_schema1-users-jb-1-Filter.db
-rw-r--r-- 2 cassandra cassandra   45 Apr 12 16:52 jw_schema1-users-jb-1-Index.db
-rw-r--r-- 2 cassandra cassandra  206 Apr 12 16:52 jw_schema1-users-jb-1-Data.db
-rw-r--r-- 2 cassandra cassandra   43 Apr 12 16:52 jw_schema1-users-jb-1-CompressionInfo.db
-rw-r--r-- 2 cassandra cassandra 4.3K Apr 12 16:52 jw_schema1-users-jb-1-Statistics.db
-rw-r--r-- 2 cassandra cassandra   79 Apr 12 16:52 jw_schema1-users-jb-1-TOC.txt
-rw-r--r-- 2 cassandra cassandra   59 Apr 12 16:52 jw_schema1-users-jb-1-Summary.db

If you md5sum on the data files between snapshot and the live data, they are identically match.

jason@localhost:/var/lib/cassandra/data/jw_schema1/users$ md5sum snapshots/1397292720524/*Data*
3d4351d714500417c74de6811b1eae3b  snapshots/1397292720524/jw_schema1-users.idxAge-jb-1-Data.db
a430a2d65c0a504fe3ab06344654a89a  snapshots/1397292720524/jw_schema1-users.idxLast-jb-1-Data.db
13798e1ffb5ed6a871d768399f54b125  snapshots/1397292720524/jw_schema1-users-jb-1-Data.db
jason@localhost:/var/lib/cassandra/data/jw_schema1/users$ md5sum *Data*
3d4351d714500417c74de6811b1eae3b  jw_schema1-users.idxAge-jb-1-Data.db
a430a2d65c0a504fe3ab06344654a89a  jw_schema1-users.idxLast-jb-1-Data.db
13798e1ffb5ed6a871d768399f54b125  jw_schema1-users-jb-1-Data.db

A snapshot made is not meaningful if you cannot restore back to the node. So from this point on ward, we will take a look on how to restore the snapshot back into the node.

Surprisingly, to restore the command, you would expect for example, nodetool restore backup, but it is not. Rather, there are a few ways to restore the given snapshot sstables.

1. You can use sstableloader,


2. copy the sstables into <data_file_directories>/data/jw_schema1/users/ and refresh by calling loadNewSSTables via jconsole or using nodetool refresh


3. use a node restart method.

It sounds like a lot of works to use either of the first two methods, I'm gonna just try on the last method way of restoring the snapshot sstables.

In order to simulate our backup will be successful, we are going to do a few simulation (disk failure, accidentally delete) here.

1. copy the snapshot backup somewhere else.


2. shutdown cassandra and delete cassandra directory.

Okay, let's continue the setup simulation environment

jason@localhost:/var/lib/cassandra/data/jw_schema1/users$ cp -r snapshots/ ~/cassandra/
jason@localhost:/var/lib/cassandra/data/jw_schema1/users$ 

jason@localhost:/var/lib/cassandra/data/jw_schema1/users$ sudo /etc/init.d/cassandra stop
jason@localhost:/var/lib/cassandra/data/jw_schema1/users$

jason@localhost:/var/lib/cassandra/data/jw_schema1/users$ cd /var/lib/cassandra/commitlog/
jason@localhost:/var/lib/cassandra/commitlog$ ls
total 2.6M
-rw-r--r-- 1 cassandra cassandra 32M Apr 11 18:52 CommitLog-3-1397213531634.log
-rw-r--r-- 1 cassandra cassandra 32M Apr 12 18:38 CommitLog-3-1397213531633.log
jason@localhost:/var/lib/cassandra/commitlog$ sudo rm -rf *
jason@localhost:/var/lib/cassandra/commitlog$ cd ../data/jw_schema1/users/
jason@localhost:/var/lib/cassandra/data/jw_schema1/users$ sudo rm -rf *
jason@localhost:/var/lib/cassandra/data/jw_schema1/users$

So we have copied the snapshot to a cassandra directory under home directory and also stop cassandra, remove all commitlog and table users in keyspace jw_schema1. Note that in this case, the schema for table users is still exists as the schema is stored in the system keyspace.

And now we will copy the snapshot from home directory back into cassandra.

jason@localhost:/var/lib/cassandra/data/jw_schema1/users$ sudo cp -r ~/cassandra/snapshots/1397292720524/jw_schema1-users* .
jason@localhost:/var/lib/cassandra/data/jw_schema1/users$ ls
total 96K
-rw-r--r-- 1 root root   76 Apr 12 18:50 jw_schema1-users.idxAge-jb-1-Data.db
-rw-r--r-- 1 root root   43 Apr 12 18:50 jw_schema1-users.idxAge-jb-1-CompressionInfo.db
-rw-r--r-- 1 root root   16 Apr 12 18:50 jw_schema1-users.idxAge-jb-1-Filter.db
-rw-r--r-- 1 root root   54 Apr 12 18:50 jw_schema1-users.idxAge-jb-1-Index.db
-rw-r--r-- 1 root root 4.3K Apr 12 18:50 jw_schema1-users.idxAge-jb-1-Statistics.db
-rw-r--r-- 1 root root   68 Apr 12 18:50 jw_schema1-users.idxAge-jb-1-Summary.db
-rw-r--r-- 1 root root   79 Apr 12 18:50 jw_schema1-users.idxAge-jb-1-TOC.txt
-rw-r--r-- 1 root root   43 Apr 12 18:50 jw_schema1-users.idxLast-jb-1-CompressionInfo.db
-rw-r--r-- 1 root root   87 Apr 12 18:50 jw_schema1-users.idxLast-jb-1-Data.db
-rw-r--r-- 1 root root   16 Apr 12 18:50 jw_schema1-users.idxLast-jb-1-Filter.db
-rw-r--r-- 1 root root   58 Apr 12 18:50 jw_schema1-users.idxLast-jb-1-Index.db
-rw-r--r-- 1 root root 4.3K Apr 12 18:50 jw_schema1-users.idxLast-jb-1-Statistics.db
-rw-r--r-- 1 root root   79 Apr 12 18:50 jw_schema1-users.idxLast-jb-1-TOC.txt
-rw-r--r-- 1 root root   75 Apr 12 18:50 jw_schema1-users.idxLast-jb-1-Summary.db
-rw-r--r-- 1 root root   43 Apr 12 18:50 jw_schema1-users-jb-1-CompressionInfo.db
-rw-r--r-- 1 root root  206 Apr 12 18:50 jw_schema1-users-jb-1-Data.db
-rw-r--r-- 1 root root 4.3K Apr 12 18:50 jw_schema1-users-jb-1-Statistics.db
-rw-r--r-- 1 root root   45 Apr 12 18:50 jw_schema1-users-jb-1-Index.db
-rw-r--r-- 1 root root   16 Apr 12 18:50 jw_schema1-users-jb-1-Filter.db
-rw-r--r-- 1 root root   79 Apr 12 18:50 jw_schema1-users-jb-1-TOC.txt
-rw-r--r-- 1 root root   59 Apr 12 18:50 jw_schema1-users-jb-1-Summary.db

So far it looks good, now if you tail the cassandra system.log and start cassandra, notice that the sstables are being read. If within these down time, data supposed to be own by this node is missed, you should by now run nodetool repair to make sure data is sync.

 INFO [main] 2014-04-12 18:52:32,555 ColumnFamilyStore.java (line 254) Initializing jw_schema1.users
 INFO [SSTableBatchOpen:1] 2014-04-12 18:52:32,568 SSTableReader.java (line 223) Opening /var/lib/cassandra/data/jw_schema1/users/jw_schema1-users-jb-1 (206 bytes)
 INFO [main] 2014-04-12 18:52:32,701 ColumnFamilyStore.java (line 254) Initializing jw_schema1.users.idxLast
 INFO [SSTableBatchOpen:1] 2014-04-12 18:52:32,719 SSTableReader.java (line 223) Opening /var/lib/cassandra/data/jw_schema1/users/jw_schema1-users.idxLast-jb-1 (87 bytes)
 INFO [main] 2014-04-12 18:52:32,802 ColumnFamilyStore.java (line 254) Initializing jw_schema1.users.idxAge
 INFO [SSTableBatchOpen:1] 2014-04-12 18:52:32,810 SSTableReader.java (line 223) Opening /var/lib/cassandra/data/jw_schema1/users/jw_schema1-users.idxAge-jb-1 (76 bytes)

jason@localhost:~/$ nodetool -h localhost repair jw_schema1 users
[2014-04-12 18:59:57,477] Starting repair command #1, repairing 1280 ranges for keyspace jw_schema1
..
[2014-04-12 19:00:50,800] Repair command #1 finished

Now we will check our data if it is still there.

jason@localhost:~/$ cqlsh 192.168.0.2 9160 -k jw_schema1
Connected to just4fun at 192.168.0.2:9160.
[cqlsh 4.1.1 | Cassandra 2.0.6 | CQL spec 3.1.1 | Thrift protocol 19.39.0]
Use HELP for help.
cqlsh:jw_schema1> select * from users;

 user_id | age | first | last     | middle
---------+-----+-------+----------+--------
       3 |  34 |  john |    smith |     a
       2 |  35 |  olee |    smith |     b
       1 |  33 |   dan |    bar   |     c
(3 rows)

cqlsh:jw_schema1>

All good. :)

In my humble opinion, because cassandra is built with durable and fault tolerant in mind, snapshot is rather not actually needed. Sure, it is fair to argue if someone deleted the data accidentally, but if you can prevent that by blocking from front end, you can actually save a lot of cost in term of cluster backup and restore maintenance cost. If you want to really ensure the data is save, spin up another cluster in another data centre, then the data is guaranteed safe from disaster. But hey, no harm learning a new tools in case you might need it later down the road.