We host a Magento webshop hosted by Amazon EC2, and we have speed issues. We're currently looking at moving to another provider. We found one hosting provider where we have a test account now. Speed is better. They use memcached, and this made me wonder how that will improve things for us, if we use that on our servers. I've read that memcached caches database tables, but I understand that it is installed on the webserver, not the database server. Are there any downsides or risks when using Memcached? Will this have unforseen effects on other applications, besides improved speed? How about RAM - will we need more memory for it to work properly? Now we have 1715MB RAM (strange number but this is what it reports). 
Memcached on a webserver
centos;cache;amazon ec2
One installs memcached to the compute node or webserver -- that way it can cache queries before they'd have to hit the wire.  Your apache instance will be faster if (eg) php can hit a rich cache before going out on networking.  It's better to devote spare memory on the db box to the db instance anyway, so that it can use as much mem as possible and buffer what it can, and that way keep it in memory.Memcached will use memory on the webserver, of course, and you don't want to over-allocate so that your compute/web node starts eating into swap:  you kill all your space tradeoffs when you do that.In my limited experience, it's not quite a magic pill; and will need tuning and watching like any other cache or with any other performance tuning effort.  It gets much better when there are more than one web host per db, but it should still show you some nice improvements with one web box and one db box.  I haven't installed it in a case where the db and web are on the same box, I should say, and if you're thinking of doing so I'd maybe reconsider.