I need to setup an in memory storage system for around 10 GB of data, consisting of many 100 kb single files(images). There will be lots of reads and fairly periodic writes(adding new files, deleting some old ones).
Now, I know that tmpfs behaves like a regular file system for which you can, for example, check free/used space with df, which is a nice feature to have. However I'm interested if ramfs would offer some advantages with regards to speed of IO operations.
I know that I can not control the size of consumed memory when using ramfs and that my system can hang if it completely consumes the free RAM, but that will not be an issue in this scenario.
To sum it up, I'm interested:
– Performance wise, which is faster: ramfs or tmpfs(and possibly why)?
– When does tmpfs use swap space? Does it move already saved data to swap(to free RAM for other programs currently running) or only new data if at that moment there is no free RAM left?
Best Answer
My recommendation:
Measure and observe real-life activity under normal conditions.
Those files are unlikely to be ALL be needed and served from cache at all times. But there's a nice tool called vmtouch that can tell you what's in cache at a given moment. You can also use it to lock certain directories or files into cache. So see what things look like after some regular use. Using tmpfs and ramfs are not necessary for this situation.
See: http://hoytech.com/vmtouch/
I think you'll be surprised to see that the most active files will probably be resident in cache already.
As far as tmpfs versus ramfs, there's no appreciable performance difference. There are operational differences. A real-life use case is Oracle, where ramfs was used to allow Oracle to manage data in RAM without the risk of it being swapped. tmpfs data can be swapped-out under memory pressure. There are also differences in resizing and modifying settings on the fly.