Getting the percentage of used space and used inodes in a mount

Question

I need to calculate the percentage of used space and used inodes for a mount path (e.g. /mnt/mycustommount) in Go.

This is my attempt:

var statFsOutput unix.Statfs_t

err := unix.Statfs(mount_path, &statFsOutput)
if err != nil {
    return err
}

totalBlockCount := statFsOutput.Blocks // Total data blocks in filesystem
freeSystemBlockCount = statFsOutput.Bfree // Free blocks in filesystem
freeUserBlockCount = statFsOutput.Bavail // Free blocks available to unprivileged user

Now the proportion I need would be something like this:

x : 100 = (totalBlockCount - free[which?]BlockCount) : totalBlockCount

i.e. x : 100 = usedBlockCount : totalBlockCount . But I don't understand the difference between Bfree and Bavail (what's 'unprivileged' user go to do with filesystem blocks?).

For inodes my attempt:

totalInodeCount = statFsOutput.Files
freeInodeCount = statFsOutput.Ffree
// so now the proportion is
// x : 100 = (totalInodeCount - freeInodeCount) : totalInodeCount

How to get the percentage for used storage? And is the inodes calculation I did correct?

Answer 1

Your comment expression isn't valid Go, so I can't really interpret it without guessing. With guessing, I interpret it as correct, but have I guessed what you actually mean, or merely what I think you mean? In other words, without showing actual code, I can only imagine what your final code will be. If the code I imagine isn't the actual code, the correctness of the code I imagine you will write is irrelevant.

That aside, I can answer your question here:

(what's 'unprivileged' user go to do with filesystem blocks?)

The Linux statfs call uses the same fields as 4.4BSD. The default 4.4BSD file system (the one called the "fast file system") uses a blocks-with-fragmentation approach to allocate blocks in a sort of stochastic manner. This allocation process works very well on an empty file system, and continues to work well, without extreme slowdown, on somewhat-full file systems. Computerized modeling of its behavior, however, showed pathological slowdowns (amounting to linear search, more or less) were possible if the block usage exceeded somewhere around 90%.

(Later, analysis of real file systems found that the slowdowns generally did not hit until the block usage exceeded 95%. But the idea of a 10% "reserve" was pretty well established by then.)

Hence, if a then-popular large-size disk drive of 400 MB¹ gave 10% for inodes and another 10% for reserved blocks, that meant that ordinary users could allocate about 320 MB of file data. At that point the drive was "100% full", but it could go to 111% by using up the remaining blocks. Those blocks were reserved to the super-user though.

These days, instead of a "super user", one can have a capability that can be granted or revoked. However, these days we don't use the same file systems either. So there may be no difference between bfree and bavail on your system.

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¹Yes, the 400 MB Fujitsu Eagle was a large (in multiple senses: it used a 19 inch rack mount setup) drive back then. People are spoiled today with their multi-terabyte SSDs. 😀