Sunday, July 1, 2012

memcached: dump to disk


Memcached is well-known, excellent memory storage. But what if you need to dump it's content to disk? This may be need, for example, in the following case: you have memcached with around 50% hitrate. Your service' average load is about 70% in rush hour. So, if your cache-server will reboot, you'll lose your cache, and requests will suddenly double, your users will suffer for this time due to timeouts.
Sounds realistic for you? Then, try this fork of memcached: memcached-dd.
As said in README, usage is straightforward: just add `-F file' option to command-line. Memcached will read this `file' at start and write to file.tmp when SIGUSR2 received. Then (after successfull write and sync), it will rename file.tmp -> file. So, `file' should be never truncated. For example:
$ memcached -F /tmp/memcache.dump -m 64 -p 11211 -l

Some notes to be clear

  • Dump performs in separate thread and doesn't block memcached itself
  • If you using TTL for your data, being restored the data will have the same TTL as in the time of dump.
  • All expired and flushed (flush_all command) content left behind
  • There is no any schedule-like maintaining for dumps, it's better to do with crontab and/or your own scripts


I'll show the usage with Perl script. Assume, you have downloaded and built memcached-dd; see INSTALLATION section in README if in doubt. This Perl-scenario will load fake data into memcached:
use Cache::Memcached;

$memd = new Cache::Memcached {
    'servers' => [ "" ]

$| = 1;

for (my $i = 0; $i <= 10000; $i++) {
    $memd->set( "key_$i", "x"x100 . " [$i]" );
    # my $val = $memd->get( "xkey_$i");

    if ($i % 1000 == 0) {
        print "\r$i...";
Having this script, launch the memcached-dd:
$ ./memcached -P /tmp/ -F /tmp/memcached.dump -m 128 -p 11211 -l
# now load the data into memcached:
$ perl ./
# and now, assuming memcached has this data, dump it:
$ kill -USR2 `cat /tmp/`
1Mb dumped: 10001 items (0 expired during dump, 0 nuked by flush)
Moving temprorary /tmp/memcached.dump.tmp -> /tmp/memcached.dump

OK, now you have file /tmp/memcached.dump with all 10000 records dumped. You may reload it anytime launching memcached-dd with the same -F (assuming you killed memcached):

$ ./memcached -P /tmp/ -F /tmp/memcached.dump -m 128 -p 11211 -l
Now check this keys with netcat:
$ echo get key_1 | nc 11211
VALUE key_1 0 104
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx [1]

As you can see, content successfully restored from dump. Hope this will help for your particular usecase. If you have some problems with memcached-dd, feel yourself free to post this issue

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