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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?

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Um, how much RAM is in the server? –  ewwhite Apr 19 at 18:01
The server has a total of 16 GB of RAM. Also worth noting is that I do not have an SSD storage but a single 7200 rpm HDD. That is why I'm considering using some type of RAM storage. –  Ivan Kovacevic Apr 19 at 18:03

4 Answers 4

up vote 7 down vote accepted

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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Awesome little utility! +1 –  Janne Pikkarainen Apr 24 at 11:48

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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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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Your answer is on the right track. However I would not agree with your conclusion regarding performance, your tests show that there ARE differences of 0.2 GB/s and 0.1 GB/s in favour of ramfs. I believe this should be tested even further to provide a valid statistical sample. Regarding 2) Yes that is known, however I wished I could get a better insight of exactly when is swap used. –  Ivan Kovacevic Apr 24 at 12:49
if we did this benchmark a bunch of times with different size files, I don't think we'll see a difference. you'll notice that when I bumped the size up four times, the difference actually narrowed rather than widening. –  Michael Martinez Apr 24 at 16:52
What about the case where you have a bunch of small files. For example writing million 100-200 kb files. Also do you get 0.2 GB/s difference repeatedly for the same file size? Which would definitely point out to performance difference. I will probably test this myself when it's on my schedule. But that is why I asked here, so I could maybe cross it from the to-do list if anyone else already did it. –  Ivan Kovacevic Apr 24 at 17:27
yeah the only way to know for sure is to do the tests. –  Michael Martinez Apr 24 at 17:35

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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My system currently has 16 GB of RAM. Its a plain Debian install running Nginx to serve those images. I have a 1 Gbit network connection which will be under 100% load all the time, serving those images at no particular order. Do you think that the kernel will load all those 10 gigs of images anyway in the cache in this scenario? –  Ivan Kovacevic Apr 19 at 17:53
Yes, if there is sufficient RAM in the system, and other applications on the server are not competing for RAM resources, those files will remain in cache. –  EEAA Apr 19 at 18:14
In that case you are probably right that this is unnecessary in my case. However I'm gonna leave the question open for a while still, just for the sake of argument ramfs vs tmpfs, in the scenario where RAM storage would be useful... I suppose such scenario exists?! –  Ivan Kovacevic Apr 19 at 18:20
I've been at unix administration for ~15 years, and I've never run into a situation where tmpfs/ramfs would have provided any benefit over the native kernel fs cache. That's not to say that situations don't exist out there where they would be warranted, but they're quite rare. Typically if you need RAM cache for things, one uses a purpose-build caching layer (Redis/Memcache/etc.). –  EEAA Apr 19 at 18:22
Disk caching will certainly work for the case where the images need to be read, but tmpfs or ramfs could still be useful if you wish to speed up a lot of random/small writes but are bound to a disk that is slow with random I/O. Do keep in mind that if the machine crashes or suffers a power failure, the contents of tmpfs will be gone since they where (only) in memory. –  Martijn Apr 22 at 22:19

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