问题
I am frankly stumped. This is beyond my experience.
I have a C# MVC program that generates a zip file in a MemoryStream for downloading. The action method is called by a button click to JavaScript.
The only problem is that in some cases the potential file size can easily exceed one Gig and from my reading, that is a common problem. I've tried upping the Maximum Allowed Content Length to 3000000000 in Request Filtering on IIS (IIS8). I've tried adding requestLimits maxAllowedContentLength to my web.config. I've even tried breaking up the zip through multiple calls to the action method (without success), although I have yet to get any confirmation/denial that this is even possible.
Is there any setting within IIS or my web.config that I could be overlooking? Could this be a company network issue, not solvable on an app developer's level?
回答1:
Okay, so it's kind of hard to explain big concepts in 400 characters or less, so I think I'm just causing more confusion sticking in the comments section. Besides, I think we're close enough here to an "answer" as you're likely to get.
The default constructor of MemoryStream essentially sets the initial size to 0. In reality, the initial size is set to somewhere around 256, but since the initial size is mostly a guide, and it doesn't actually claim that space until its needed, it starts at 0.
Each time you write to the stream, it checks how much is being written versus the remaining size of the buffer array. If it can't fit the write, it creates a new, larger buffer array and copies the old buffer array into that. In this way, setting an initial size can help somewhat, in that you start off with a larger initial buffer array and you may not need to grow that buffer. You might have a better chance of getting a contiguous block of memory, which I'll explain the importance of in a bit, but that actually kind of works against you, as well. If you only need 1MB for the file, but you're initializing with 100MB and there's not 100MB of contiguous memory, you'll get an OutOfMemoryException, even though there might be 1MB of contiguous memory available.
Regardless of whether you initialize or not, there remains certain immutable facts. First, MemoryStream requires contiguous memory. Even if you technically have memory available on the system, it's possible you might not have large blocks of available memory. In other words if you have 4GB available, but it's all fragmented, even trying to create a 1GB stream in memory could fail, simply because it can't reserve 1GB of contiguous memory. Obviously, the larger the file you're tying to create in memory, the greater the chances that you're going to run into this issue. For this reason alone, I would say you're out of luck without raising the amount of system RAM. With 8GB and probably only 4-6GB actually available to IIS and then split up between worker processes and threads, the odds that you're going to be able to claim 25% or so of the available RAM as contiguous space, is highly unlikely.
The next immutable fact may or may not be relevant, but since you haven't specified, I'll mention it. If your web app is deployed as 32-bit, you'll have a hard limit of 2GB for any object, meaning a MemoryStream could never house more than 2GB (actually around 1.3-1.6GB as .NET code consumes some of that address space), and any attempt to make it do so will result in an OutOfMemoryException, even if you had some ridiculous amount of RAM on the system like 1TB+. If your app is 64-bit, this is less likely an issue as you can address a ton more memory, assuming it's compiled properly. You'd have to pretty much try to screw that up, though, so you should be fine.
Finally, multiple writes can cause an issue as well. As I said previously, the buffer array resizes (if necessary) in response to writes. Each time it resizes, the new buffer array must also be able to fit in contiguous address space. As a result, multiple resizes can cause you to bump into an OutOfMemoryException you wouldn't have hit if you had written all the data from the start. This is where initializing the MemoryStream can be helpful, but as I said before, it's also a double-edged sword, as your initial buffer size might be too great to begin with and you end up with an exception where you may have not had one letting it grow organically. Long and short, try to write everything to the stream in one go rather than piecemeal.
来源：https://stackoverflow.com/questions/41961496/how-do-i-increase-maximum-download-size-in-an-mvc-application