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
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):
|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|
|Resets statistics||stats reset|
|version||Prints server version.||version|
|verbosity||Increases log level||verbosity|
|quit||Terminate telnet session||quit|
You can query the current traffic statistics using the command
statsYou will get a listing which serves the number of connections, bytes in/out and much more.
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
You can query the current memory statistics using
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 itemscommand 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 [...] ENDThis 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.