Regarding Memcached Server configuration?

[Marcus Bointon]

On 26 Sep 2007, at 18:39, Dan Christian wrote:

> I'm a bit confused about the redundancy aspects of memcached.  My
> understanding is that you can have multiple memcacheds, but they store
> different objects (based on the keys).
>
> Tell me if I understand this right.  If I run a memcached on each of 2
> machines, then the failure of a machine takes out half the cached
> objects.

Yes, it's a bit like a JBOD array of disks - losing a disk will only  
lose the data on that disk. From the client perspective there is  
(generally) only one apparent cache.

> These values will continue to be un-cached until that
> machine is removed from the client's configuration.
>
> Is there an automatic recovery mechanism?

Not for the data (it's just a cache!), but for the general operation,  
yes. Try searching for 'consistent hashing' in this list's archives.

> Is there a (big) performance hit when the configuration changes?

Depends on your app - the more memcache servers you have, the smaller  
the hit if one dies. If you have a config change that affects all  
servers, it can amount to a big hit. You can cheat a bit by 'pre- 
warming' your newly configured caches. It's also relative to how much  
more efficient the cache is than your app.

Marcus
-- 
Marcus Bointon
Synchromedia Limited: Creators of http://www.smartmessages.net/
UK resellers of info at hand CRM solutions
marcus at synchromedia.co.uk | http://www.synchromedia.co.uk/