memcached Cheat Sheet

This is a short summary of everything important that helps to inspect a running memcached instance. You need to know that memcached requires you to connect to it via telnet. The following post describes the usage of this interface.

How To Connect

Use "ps -ef" to find out which IP and port was passed when memcached was started and use the same with telnet to connect to memcache. Example:

telnet 10.0.0.2 11211

Supported Commands

The supported commands (the official ones and some unofficial) are documented in the doc/protocol.txt document.

Sadly the syntax description isn't really clear and a simple help command listing the existing commands would be much better. Here is an overview of the commands you can find in the source (as of 16.12.2008):

Command Description Example
get Reads a value get mykey
set Set a key unconditionally set mykey 0 60 5
add Add a new key add newkey 0 60 5
replace Overwrite existing key replace key 0 60 5
append Append data to existing key append key 0 60 15
prepend Prepend data to existing key prepend key 0 60 15
incr Increments numerical key value by given number incr mykey 2
decr Decrements numerical key value by given number decr mykey 5
delete Deletes an existing key delete mykey
flush_all Invalidate specific items immediately flush_all
Invalidate all items in n seconds flush_all 900
stats Prints general statistics stats
Prints memory statistics stats slabs
Prints memory statistics stats malloc
Print higher level allocation statistics stats items
stats detail
stats sizes
Resets statistics stats reset
version Prints server version. version
verbosity Increases log level verbosity
quit Terminate telnet session quit

Traffic Statistics

You can query the current traffic statistics using the command

stats
You will get a listing which serves the number of connections, bytes in/out and much more.

Example Output:

STAT pid 14868
STAT uptime 175931
STAT time 1220540125
STAT version 1.2.2
STAT pointer_size 32
STAT rusage_user 620.299700
STAT rusage_system 1545.703017
STAT curr_items 228
STAT total_items 779
STAT bytes 15525
STAT curr_connections 92
STAT total_connections 1740
STAT connection_structures 165
STAT cmd_get 7411
STAT cmd_set 28445156
STAT get_hits 5183
STAT get_misses 2228
STAT evictions 0
STAT bytes_read 2112768087
STAT bytes_written 1000038245
STAT limit_maxbytes 52428800
STAT threads 1
END

Memory Statistics

You can query the current memory statistics using

stats slabs

Example Output:

STAT 1:chunk_size 80
STAT 1:chunks_per_page 13107
STAT 1:total_pages 1
STAT 1:total_chunks 13107
STAT 1:used_chunks 13106
STAT 1:free_chunks 1
STAT 1:free_chunks_end 12886
STAT 2:chunk_size 100
STAT 2:chunks_per_page 10485
STAT 2:total_pages 1
STAT 2:total_chunks 10485
STAT 2:used_chunks 10484
STAT 2:free_chunks 1
STAT 2:free_chunks_end 10477
[...]
STAT active_slabs 3
STAT total_malloced 3145436
END

If you are unsure if you have enough memory for your memcached instance always look out for the "evictions" counters given by the "stats" command. If you have enough memory for the instance the "evictions" counter should be 0 or at least not increasing.

Which Keys Are Used?

There seems to be no builtin function to determine the currently set keys. However you can use the

stats items
command to determine how many keys do exist.
stats items
STAT items:1:number 220
STAT items:1:age 83095
STAT items:2:number 7
STAT items:2:age 1405
[...]
END
This at least helps to see if any keys are used. To dump the key names from a PHP script that already does the memcache access you can use the PHP code from 100days.de.

Never Set a Timeout > 30 Days!

While this has nothing to do with the telnet access this is a problem you might run into. If you try to "set" or "add" a key with a timeout bigger than the allowed maximum you might not get what you expect because memcached then treats the value as a Unix timestamp. Also if the timestamp is in the past it will do nothing at all. Your command will silently fail.

So if you want to use the maximum lifetime specify 2592000. Example:

set my_key 0 2592000 1
1

Disappearing Keys on Overflow

Despite the documentation saying something about wrapping around 64bit overflowing a value using "incr" causes the value to disappear. It needs to be created using "add"/"set" again.

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