CheatMemcached In-memory cache
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Start here
memcached -m 256 -l 127.0.0.1
Run with 256 MB, local only
nc <host> 11211
Open a text session
set <key> <flags> <ttl>
<bytes>
Store a value
get <key>Read it back
delete <key>
Remove it
incr <key> <n>
Atomic counter
statsHit rate, memory, evictions
flush_allInvalidate everything
Concepts
- Client hashes key to a server
- memcached slabs + LRU
- Database source of truth
- memcached next read hits
Run the daemon
memcached -m <MB>
Set item memory (default 64)
Not a hard cap; real usage runs a bit higher. Check maxbytes
in stats settings.
memcached -p <port>
TCP port (default 11211)
memcached -l <addr>
Listen on one interface
Bind to 127.0.0.1 or a private address, never a public one.
memcached -d -u <user>
Daemonize, drop to a user
-u only applies when started as root.
memcached -c <n>
Max connections (default 1024)
memcached -t <n>
Worker threads (default 4)
memcached -v / -vv
Log errors / also commands
memcached -s <path>
Listen on a UNIX socket
Disables TCP. Combine with -a <mask> for permissions.
memcached -U <port>
Enable UDP (off by default)
memcached -MError on full instead of evicting
memcached -I <size>
Max item size (default 1m)
memcached -P <file>
Write a pid file (with -d)
memcached -FDisable flush_all
memcached -AEnable the shutdown command
memcached -B ascii
Force the text protocol
memcached -o <opt>,<opt>
Extended options
memcached -VPrint version
Talk to it
nc <host> <port>
Open a raw text session
Type commands ending in Enter; nc usually sends \n, which
memcached accepts.
printf '<cmd>\r\n' | nc -q1 <host>
<port>
Send one command
-q1 is the traditional netcat option; OpenBSD nc uses -N.
telnet <host> <port>
Same, with telnet
versionShow server version
verbosity <n>
Change log level at runtime
quitClose the session
Store and fetch
set <key> <flags> <ttl>
<bytes>
Store, overwrite if present
The value goes on the next line and must be exactly
<bytes> long. Reply: STORED.
get <key> [<key> ...]
Fetch one or more keys
Misses are simply absent from the reply; END closes it.
delete <key>
Remove a key
add <key> <flags> <ttl>
<bytes>
Store only if absent
Replies NOT_STORED if the key exists; useful as a simple lock.
replace <key> <flags> <ttl>
<bytes>
Store only if present
append <key> 0 0 <bytes>
Add bytes after the value
prepend <key> 0 0 <bytes>
Add bytes before the value
gets <key>
Fetch with a CAS id
cas <key> <flags> <ttl> <bytes>
<cas>
Store only if unchanged
EXISTS means someone changed it since your gets.
touch <key> <ttl>
Update expiry only
gat <ttl> <key>
Get and set new expiry
gats <ttl> <key>
Get, touch, return CAS
Counters
incr <key> <n>
Add n to a number
The value must be a decimal string; replies with the new
number.
decr <key> <n>
Subtract n, floor at 0
set <key> 0 0 1
Seed a counter with 0
Then send 0 on the next line. incr on a missing key gives
NOT_FOUND.
Meta commands
mg <key> v
Meta get: return value
mg <key> v t f c k
Value plus ttl, flags, CAS, key
Each flag adds a field to the reply; t is -1 for no expiry.
ms <key> <bytes> T<ttl>
Meta set with a ttl
md <key>Meta delete
mnNo-op, marks end of a pipeline
mg <key> s v q
Quiet get: no reply on miss
ma <key>Meta arithmetic (incr)
Stats and inspection
statsGeneral counters
stats settingsEffective config of the daemon
stats itemsPer-slab item counts and age
stats slabsPer-slab memory use
stats sizesItem size histogram
Needs -o track_sizes on current versions.
stats connsOpen connections
stats resetZero resettable counters
lru_crawler metadump all
List keys with metadata
No plain keys command exists; this is the supported way.
Disabled by -X.
watchStream live log of requests
stats cachedump <slab> <n>
Dump keys of a slab
Legacy; limited output.
flush_all [<delay>]
Invalidate all items
Items are only marked stale, memory is not freed immediately.
shutdownStop the server
Key stats to read
| Field | Look for |
|---|---|
get_hits / get_misses |
hits / (hits+misses) |
evictions |
rising: too little RAM |
curr_items |
items now |
bytes / limit_maxbytes |
used vs. limit |
curr_connections |
clients now |
listen_disabled_num |
non-zero: -c hit |
Recipes
printf 'set k 0 60 5\r\nhello\r\nget k\r\nquit\r\n' | nc -q1 127.0.0.1 11211printf 'stats\r\nquit\r\n' | nc -q1 127.0.0.1 11211 | awk '/get_hits/{h=$3} /get_misses/{m=$3} END{print h/(h+m)}'printf 'lru_crawler metadump all\r\nquit\r\n' | nc -q1 127.0.0.1 11211 | cut -d' ' -f1memcached -m 64 -l 127.0.0.1 -p 11311 -vv
