The article was significantly cut down for brevity, so any detail of how to find the right balance is beyond the scope :)
The original question was that it can be really hard to determine how much adding RAM will offload from the disk. IE: with extstore recent objects are served by RAM and never disk, so they don't count against your IO balance. If you're coming from a RAM backed system that has 500k requests per second, it's going to take some creative introspection to figure out how much RAM you'd need to keep the disk accesses below 400k, or whatever your target is.
Along those lines I just suggested a simple experiment. which is to just take an existing fully specced instance, add a disk, and test a few RAM settings to see how it looks. Most actual users of extstore have just done that since they're extending/replacing an existing cluster.
There're actually other ways you can simply read the numbers out of memcached but it just takes more effort or familiarity with its internals.
The original question was that it can be really hard to determine how much adding RAM will offload from the disk. IE: with extstore recent objects are served by RAM and never disk, so they don't count against your IO balance. If you're coming from a RAM backed system that has 500k requests per second, it's going to take some creative introspection to figure out how much RAM you'd need to keep the disk accesses below 400k, or whatever your target is.
Along those lines I just suggested a simple experiment. which is to just take an existing fully specced instance, add a disk, and test a few RAM settings to see how it looks. Most actual users of extstore have just done that since they're extending/replacing an existing cluster.
There're actually other ways you can simply read the numbers out of memcached but it just takes more effort or familiarity with its internals.