[PATCH] D39464: Define fs::allocate_file which preallocates disk blocks.

[Rui Ueyama via Phabricator via llvm-commits]

ruiu added inline comments.


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Comment at: llvm/lib/Support/Unix/Path.inc:443
+#ifdef HAVE_FALLOCATE
+  if (fallocate(FD, 0, 0, Size) == -1)
+    return std::error_code(errno, std::generic_category());
----------------
kettenis wrote:
> mgorny wrote:
> > ruiu wrote:
> > > mgorny wrote:
> > > > Any reason not to use `posix_fallocate()` which is more portable by definition?
> > > `posix_fallocate` does not fail even on filesystems that do not support block preallocation. What it does (at least in glibc) when the underlying filesystem doesn't have the feature is to write a zero byte for each disk block to force the filesystem to actually allocate disk blocks. It is as you can imagine pretty slow.
> > Ok then.
> Can I ask you to reconsider this?  fallocate(2) is Linux-specific and other systems are likely to only implement the standardized posix_fallocate(2).  And while the posix_fallocate(2) implementation might be somewhat suboptimal on filesystems that don't implement fallocate(2), this should only affect the small number of users that use lld on non-standard filesystems.
> 
> When I said that we might consider implementing fallocate(2) on OpenBSD, I really meant to say that we will consider implementing posix_fallocate(2).
I don't know if we really want to hide fallocate and posix_fallocate because the performance characteristics of the two functions are so different when they are used on a file system that doesn't support block preallocation. I believe posix_fallocate shouldn't have completely abstracted away that difference.

That said I understand your concern too. This code might be too specific to Linux, and it just does nothing for other Unix systems. That's bad.

How about this: we can make this function to call posix_fallocate and make FileOutputBuffer to use this function only when the disk is almost full. Then in most cases we don't need to call posix_fallocate at all on any system, and it can still catch disk full errors in practice.


https://reviews.llvm.org/D39464