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Ivan Kovacevic
Asked: 2014-04-20 09:44:24 +0800 CST

Performance difference between ramfs and tmpfs

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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?

linux

4 Answers

  1. 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.

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  2. Don't over-think this. Put enough RAM in your system and let the kernel's disk cache take care of things for you. That way you get the benefit of reads coming directly from memory, while still being able to persist data on disk.

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  3. 1) Performance benchmark.

    Using this page as a reference, I did I/O comparison between tmpfs and ramfs, and the results are that it is pretty much identical in terms of performance:

    # !mount
mount | grep -E "tmp|ram"
tmpfs on /dev/shm type tmpfs (rw)
ramfs on /mnt/ram type ramfs (rw,size=1G)

# dd bs=1M count=1024 if=/dev/zero of=/dev/shm/test conv=fdatasync
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 0.634054 s, 1.7 GB/s

# dd bs=1M count=1024 if=/dev/zero of=/mnt/ram/test conv=fdatasync
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 0.559557 s, 1.9 GB/s

# dd bs=1M count=4096 if=/dev/zero of=/dev/shm/test conv=fdatasync
4096+0 records in
4096+0 records out
4294967296 bytes (4.3 GB) copied, 2.5104 s, 1.7 GB/s

# dd bs=1M count=4096 if=/dev/zero of=/mnt/ram/test conv=fdatasync
4096+0 records in
4096+0 records out
4294967296 bytes (4.3 GB) copied, 2.36923 s, 1.8 GB/s

    2) According to this page, tmpfs uses swap, and ramfs does not use swap.

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  4. If you have a sufficient amount of RAM installed to host the various kernel buffers, the applications stack and heaps, the regular file system cache and all the files you intent to put in it, ramfs should never be slower than tmpfs as there will be no risk of physical I/O by design. Physical I/Os are undoubtedly the main cause of performance degradation in that area.

    However, if you have not that amount of RAM installed, using ramfs might and probably will be slower than tmpfs as the latter is using the virtual memory heuristic to decide what should better be on disk (i.e. in the swap area) vs what should be on RAM while with tmpfs, your file system data is stuck on RAM which might be a waste of resource.

    To answer you second question, yes, tmpfs will move old data first to the swap area, not the last "hot" one.

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