The resounding answer you will get from this list is: You don't, can't
and won't with memcached. That is not its job. It will never be its job.
Perhaps when storage engines are done, maybe. But then you won't get the
performance that you get with memcached. There is a trade off for
performance.
Brian.
--------
http://brian.moonspot.net/
On 3/12/10 3:02 PM, martin.grotzke wrote:
Hi,
I know that this topic is rather burdened, as it was said often enough
that memcached never was created to be used like a reliable datastore.
Still, there are users interested in some kind of reliability, users
that want to store items in memcached and be "sure" that these items
can be pulled from memcached as long as they are not expired.
I read the following on memcached's memory management:
"Memcached has two separate memory management strategies:
- On read, if a key is past its expiry time, return NOT FOUND.
- On write, choose an appropriate slab class for the value; if it's
full, replace the oldest-used (read or written) key with the new one.
Note that the second strategy, LRU eviction, does not check the expiry
time at all." (from "peeping into memcached", [1])
I also found "Slabs, Pages, Chunks and Memcached" ([2]) a really good
explanation of memcached's memory model.
Having this as background, I wonder how it would be possible to get
more predictability regarding the availability of cached items.
Asume that I want to store sessions in memcached. How could I run
memcached so that I can be sure that my sessions are available in
memcached when I try to "get" them? Additionally asume, that I expect
to have 1000 sessions at a time in max in one memcached node (and that
I can control/limit this in my application). Another asumption is,
that sessions are between 50kb and 200 kb.
The question now is how do I have to run memcached to "store" these
sessions in memcached?
Would it be an option to run memcached with a minimum slab size of
200kb. Then I would know that for each session a 200kb chunk is used.
When I have 1000 session between 50kb and 200kb this should take 200mb
in total. When I run memcached with more than 200mb memory, could I be
sure, that the sessions are alive as long as they are not expired?
What do you think about this?
Cheers,
Martin
[1] http://blog.evanweaver.com/articles/2009/04/20/peeping-into-memcached/
[2] http://www.mikeperham.com/2009/06/22/slabs-pages-chunks-and-memcached/
