Home of Disk Image Memory Usage

Question

I'm not sure whether this is the appropriate forum for the question, but I couldn't find a better one. I merely want to know where the memory for a mounted disk image comes from. Basically I'm wondering if it only comes from the root volume, the volume that the disk image is stored on, virtual memory, or something I haven't thought of. Thanks for any help.

Answer 1

I think you're over-analyzing it a little, but there are several facets to it, so it can be a little confusing.
For any non-sparse (I'll get to that in a minute) .dmg stored on any disk (whether that's local or network-based), it takes up the amount of disk space equal to the size of the disk image volume. That is, a 2GB .dmg takes up 2GB on disk.
In the case of a .dmg stored on a file server, it doesn't matter whether it's a 2GB .dmg, or a 2GB .jpg - as far as the file server is concerned it is just a stream of bytes.
So the file server just sees a stream of thousands (or millions) of bytes. The server doesn't know that stream of bytes, if read accordingly, maps to a disk image containing other files.
It is the host system (where you mount the disk image) which does all the work of reading the disk image headers to determine the embedded volume size, reading the directories, accessing files, etc.
As a result, the disk image takes an appropriate amount of disk space on the physical volume it's stored on, plus a small amount of memory (enough to read the disk headers and directory) on the host system when the image is mounted. It does not consume 2GB of ram because the entire .dmg is not read into memory - only enough so that the host knows what files are on the disk.
The same principle is true for compressed images, where the data is compressed within the disk image. Again, the file server doesn't know anything about the contents of the .dmg file, it's the host OS that takes care of compressing and decompressing the data as it's read from/written to the disk image. To the file server, it's just another stream of bytes.
Sparse images are a little different, so they're worth mentioning. Sparse images contain a disk header/directory of a certain size (e.g. 2GB), but only contain sufficient data for the files in it.
That is, if you create a 2GB disk image and store one 10MB file in it, it takes up 2GB on disk (that's the size of the disk image.
However, if you create a 2GB sparse disk image and store the same 10MB file on it, the disk image will take up a little more than 10MB on disk (10MB for the file plus some overhead for the size of the headers, directory, etc.).
This doesn't change the overriding principle that the host system doesn't know or care about the contents of the file, it's the host OS that takes care of all of that.