A little bit of background first. Maybe, hopefully, this can save some else some trouble and frustration. Move down to the TL;DR to move on to the actual question.
We currently have a couple genetics workflows related to gene sequencing running in azure batch. Some of which are quite light and I'd like to move them to an Azure Function running a Docker container. For this purpose I have created a docker image, based on the azure-functions image, containing anaconda with the necessary packages to run our most common, lighter workflows. My initial attempt produced a huge image of ~8GB. Moving to Miniconda and a couple other adjustments has reduced the image size to just shy of 3.5GB. Still quite large, but should be manageable.
For this function I created an Azure Function running on an App Service Plan on the P1V2 tier on the belief that I would have 250GB storage to work with as stated in the tier description:
I encountered some issues with loading my first image (the large one) after a couple fixes where the log indicated that there was no more space left on the device. This puzzled me since the quota stated that I'd used some 1.5MB of the 250 total. At this point I reduced the image size and could at least successfully deploy the image again. Enabling SSH support I logged in to the container via SSH and ran df -h.
Okay so the function does not have the advertised 250GB of storage available runtime. It only has about 34. I spent some time searching in the documentation but could not find anything indicating that this should be the case. I did find this related SO question which clarified things a bit. I also found this still open issue on the azure functions github repo. Seems that more people are having the same issue and is not aware of the local storage limitation of the SKU. I might have overlooked something so if this is in fact documented I'd be happy if someone could direct me there.
Now the reason I need some storage is that I need to get the raw data file which can be anything from a handful of MBs to several GBs. And the workflow then subsequently produces multiple files varying between a few bytes and several GBs. The intention was, however, not to store this on the function instances but to complete the workflow and then store the resulting files in a blob storage.
TL;DR
You do not get the advertised storage capacity for functions running on an App Service Plan on the local instance. You get around 20/60/80GB depending on the SKU.
I need 10-30GB of local storage temporarily until the workflow has finished and the resulting files can be stored elsewhere.
How can I reduce the spent storage on the local instance?
Finally, the actual question. You might have noticed on the screenshot from the df -h command that of the available 34GB a whopping 25GB is already used. Which leaves 7.6GB to work with. I already mentioned that my image is ~3.5GB of size. So how come there is a total of 25GBs used and is there any change at all to reduce this aside from shrinking my image? That being said, if I'd removed my image completely (freeing 3.5GB of storage) it would still not be quite enough. Maybe the function simply needs stuff worth over 20GB of storage to run?
Note: It is not a result of cached docker layers or the like since I have tried scaling the app service plan which clears the cached layers/images and re-downloads the image.
Moving up a tier gives me 60GB of total available storage on the instance. Which is enough. But it feels very overkill when I don't need the rest that this tier offers.
Attempted solution 1
One thing I have tried, which might be of help to others, is mounting a file share on the function instance. This can be done with very little effort as shown by the MS docs. Great, now I could directly write to a file share saving me some headache and finally move on. Or so I thought. While this mostly worked great it still threw an exception indicating that it ran out space on the device at some point leading me to believe that it may be using local storage as temporary storage, buffer, or whatever. I will continue looking into it and see if I can figure that part out.
Any suggestions or alternative solutions will be greatly appreciated. I might just decide to move away from Azure Functions for this specific workflow. But I'd still like to clear things up for future reference.
Thanks in advance
niknoe