Hi,
I would like to know the performances of Memcached on remote server(on same LAN) with Disk Caching.Besides Memcached is a scalable cache solution, would there be any advantage of using Memcached with respect to performance when compared to disk caching.
Regards, Mugil.
From my personal experience I've found memcached isn't as fast as disk cache. I believe this is because of the OS's disk IO's caching, but memcached allows for a "scalable" cache, meaning if you have more than 1 server accessing the same cache data, it will scale (especially since memcached have a very low CPU overhead compared to PHP). The only way to allow more than 1 machine to access a disk cache at once is network mount which is surely going to kill the speed of the accesses. Also one other thing you have to worry about with file caching is garbage collection to prevent disk from being saturated.
As your site(s) scale, you might want to change your mind later, so whatever choice you make, use a cache wrapper, so you can easily change your method. Zend provides a good API.
Reads are going to be about the same speed (since the OS will cache files frequently accessed)... The difference is going to be with writes. With memcached, all it needs to do is write the ram. But with file storage, it gets a little bit more tricky. If you have write caching enabled, it'll be about as fast. But most servers have it turned off (Unless they have a battery backed cache) for more reliable writes in the case of power failure. So if yours is not using a write cache, it'll require the write to complete on disk (can take 5+ ms on server grade drives, possibly more on desktop grade hardware) before returning. So writing files may be significantly slower than memcached.
But there's another caviat. With files, when you want to write, you'd need to lock the file yourself. This means that if two processes try to write to the same file, the writes would have to complete serially. With memcached, those two writes get pushed into a queue and happen in the order received, but the writing process (PHP) doesn't need to wait for the actual commit...