reHow memory mapped files, filesystems and cloud storage works
Kelly has an interesting post about memory mapped files and the cloud. This is in response to a comment on my post where I stated that we don’t reserve space up front in Voron because we support cloud providers that charge per storage.
From Kelly’s post, I assume she thinks about running it herself on her own cloud instances, and that is what here pricing indicates. Indeed, if you want to get a 100GB cloud disk from pretty much anywhere, you’ll pay for the full 100GB disk from day 1. But that isn’t the scenario that I actually had in mind.
I was thinking about the cloud providers. Imagine that you want to go to RavenHQ, and get a db there. You sigh up for a 2 GB plan, and all if great. Except that on the very first write, we allocate a fixed 10 GB, and you start paying overage charges. This isn’t what you pay when you run on your own hardware. This is what you would have to deal with as a cloud DBaaS provider, and as a consumer of such a service.
That aside, let me deal a bit with the issues of memory mapped files & sparse files. I created 6 sparse files, each of them 128GB in size in my E drive.
As you can see, this is a 300GB disk, but I just “allocated” 640GB of space in it.
This also shows that there has been no reservation of space on the disk. In fact, it is entirely possible to create files that are entirely too big for the volume they are located on.
I did a lot of testing with mmap files & sparseness, and I came to the conclusion that you can’t trust it. You especially can’t trust it in a cloud scenario.
But why? Well, imagine the scenario where you need to use a new page, and the FS needs to allocate one for you. At this point, it need to find an available page. That might fail, let us imagine that this fails because of no free space, because that is easiest.
What happens then? Well, you aren’t access things via an API, so there isn’t an error code it can return, or an exception to be thrown.
In Windows, it will use Standard Exception Handler to throw the error. In Linux, that will be probably generate a SIVXXX error. Now, to make things interesting, this may not actually happen when you are writing to the newly reserved page, it may be deferred by the OS to a later point in time (or if you call msync / FlushViewOfFile). At any rate, that means that at some point the OS is going to wake up and realize that it promised something it can’t deliver, and in that point (which, again, may be later than the point you actually wrote to that page) you are going to find yourself in a very interesting situation. I’ve actually tested that scenario, and it isn’t a good one form the point of view of reliability. You really don’t want to get there, because then all bets are off with regards to what happens to the data you wrote. And you can’t even do graceful error handling at that point, because you might be past the point.
Considering the fact that disk full is one of those things that you really need to be aware about, you can’t really trust this intersection of features.