If you enable verbose mode, using the
-vvv options, then the
information output by memcached includes
details of the operations being performed.
Without the verbose options, memcached normally produces no output during normal operating.
Output when using
The lowest verbosity level shows you:
Errors and warnings
Protocol and socket errors, including exhausting available connections
Each registered client connection, including the socket descriptor number and the protocol used.
32: Client using the ascii protocol 33: Client using the ascii protocol
The socket descriptor is only valid while the client remains connected. Non-persistent connections may not be effectively represented.
Examples of the error messages output at this level include:
<%d send buffer was %d, now %d Can't listen for events on fd %d Can't read from libevent pipe Catastrophic: event fd doesn't match conn fd! Couldn't build response Couldn't realloc input buffer Couldn't update event Failed to build UDP headers Failed to read, and not due to blocking Too many open connections Unexpected state %d
Output when using
When using the second level of verbosity, you get more detailed information about protocol operations, keys updated, chunk and network operatings and details.
During the initial start-up of memcached with this level of verbosity, you are shown the sizes of the individual slab classes, the chunk sizes, and the number of entries per slab. These do not show the allocation of the slabs, just the slabs that would be created when data is added. You are also given information about the listen queues and buffers used to send information. A sample of the output generated for a TCP/IP based system with the default memory and growth factors is given below:
$> memcached -vv slab class 1: chunk size 80 perslab 13107 slab class 2: chunk size 104 perslab 10082 slab class 3: chunk size 136 perslab 7710 slab class 4: chunk size 176 perslab 5957 slab class 5: chunk size 224 perslab 4681 slab class 6: chunk size 280 perslab 3744 slab class 7: chunk size 352 perslab 2978 slab class 8: chunk size 440 perslab 2383 slab class 9: chunk size 552 perslab 1899 slab class 10: chunk size 696 perslab 1506 slab class 11: chunk size 872 perslab 1202 slab class 12: chunk size 1096 perslab 956 slab class 13: chunk size 1376 perslab 762 slab class 14: chunk size 1720 perslab 609 slab class 15: chunk size 2152 perslab 487 slab class 16: chunk size 2696 perslab 388 slab class 17: chunk size 3376 perslab 310 slab class 18: chunk size 4224 perslab 248 slab class 19: chunk size 5280 perslab 198 slab class 20: chunk size 6600 perslab 158 slab class 21: chunk size 8256 perslab 127 slab class 22: chunk size 10320 perslab 101 slab class 23: chunk size 12904 perslab 81 slab class 24: chunk size 16136 perslab 64 slab class 25: chunk size 20176 perslab 51 slab class 26: chunk size 25224 perslab 41 slab class 27: chunk size 31536 perslab 33 slab class 28: chunk size 39424 perslab 26 slab class 29: chunk size 49280 perslab 21 slab class 30: chunk size 61600 perslab 17 slab class 31: chunk size 77000 perslab 13 slab class 32: chunk size 96256 perslab 10 slab class 33: chunk size 120320 perslab 8 slab class 34: chunk size 150400 perslab 6 slab class 35: chunk size 188000 perslab 5 slab class 36: chunk size 235000 perslab 4 slab class 37: chunk size 293752 perslab 3 slab class 38: chunk size 367192 perslab 2 slab class 39: chunk size 458992 perslab 2 <26 server listening (auto-negotiate) <29 server listening (auto-negotiate) <30 send buffer was 57344, now 2097152 <31 send buffer was 57344, now 2097152 <30 server listening (udp) <30 server listening (udp) <31 server listening (udp) <30 server listening (udp) <30 server listening (udp) <31 server listening (udp) <31 server listening (udp) <31 server listening (udp)
Using this verbosity level can be a useful way to check the effects of the growth factor used on slabs with different memory allocations, which in turn can be used to better tune the growth factor to suit the data you are storing in the cache. For example, if you set the growth factor to 4 (quadrupling the size of each slab):
$> memcached -f 4 -m 1g -vv slab class 1: chunk size 80 perslab 13107 slab class 2: chunk size 320 perslab 3276 slab class 3: chunk size 1280 perslab 819 slab class 4: chunk size 5120 perslab 204 slab class 5: chunk size 20480 perslab 51 slab class 6: chunk size 81920 perslab 12 slab class 7: chunk size 327680 perslab 3 ...
During use of the cache, this verbosity level also prints out detailed information on the storage and recovery of keys and other information. An example of the output during a typical set/get and increment/decrement operation is shown below.
32: Client using the ascii protocol <32 set my_key 0 0 10 >32 STORED <32 set object_key 1 0 36 >32 STORED <32 get my_key >32 sending key my_key >32 END <32 get object_key >32 sending key object_key >32 END <32 set key 0 0 6 >32 STORED <32 incr key 1 >32 789544 <32 decr key 1 >32 789543 <32 incr key 2 >32 789545 <32 set my_key 0 0 10 >32 STORED <32 set object_key 1 0 36 >32 STORED <32 get my_key >32 sending key my_key >32 END <32 get object_key >32 sending key object_key1 1 36 >32 END <32 set key 0 0 6 >32 STORED <32 incr key 1 >32 789544 <32 decr key 1 >32 789543 <32 incr key 2 >32 789545
During client communication, for each line, the initial character shows the direction of flow of the information. The < for communication from the client to the memcached server and > for communication back to the client. The number is the numeric socket descriptor for the connection.
Output when using
This level of verbosity includes the transitions of connections between different states in the event library while reading and writing content to/from the clients. It should be used to diagnose and identify issues in client communication. For example, you can use this information to determine if memcached is taking a long time to return information to the client, during the read of the client operation or before returning and completing the operation. An example of the typical sequence for a set operation is provided below:
<32 new auto-negotiating client connection 32: going from conn_new_cmd to conn_waiting 32: going from conn_waiting to conn_read 32: going from conn_read to conn_parse_cmd 32: Client using the ascii protocol <32 set my_key 0 0 10 32: going from conn_parse_cmd to conn_nread > NOT FOUND my_key >32 STORED 32: going from conn_nread to conn_write 32: going from conn_write to conn_new_cmd 32: going from conn_new_cmd to conn_waiting 32: going from conn_waiting to conn_read 32: going from conn_read to conn_closing <32 connection closed.
All of the verbosity levels in memcached are
designed to be used during debugging or examination of issues.
The quantity of information generated, particularly when using
-vvv, is significant, particularly on a busy
server. Also be aware that writing the error information out,
especially to disk, may negate some of the performance gains you
achieve by using memcached. Therefore, use in
production or deployment environments is not recommended.