> I suspect that this completely defeats the purpose of pre-allocating space.
There are two reasons to preallocate. One is so that you can get by with fdatasync rather than full fsync (that is, you don't have to sync file metadata as well, which is usually an extra seek; file length is the most commonly changed part of "metadata").
The other is to use mmap, since you can't change the size of an mmap'd file. This is the only part that mongodb cares about, since they never fsync.
There may be reasons to mmap a sparse file but I can't think of any.
There may be reasons to mmap a sparse file but I can't think of any.
It sounds like you understand the forces at work here, but I am confused by your statements. Do you think that "pre-allocating" a sparse file is a valid alternative to writing 2GB of zeros to disk in this case?
There are two reasons to preallocate. One is so that you can get by with fdatasync rather than full fsync (that is, you don't have to sync file metadata as well, which is usually an extra seek; file length is the most commonly changed part of "metadata").
The other is to use mmap, since you can't change the size of an mmap'd file. This is the only part that mongodb cares about, since they never fsync.
There may be reasons to mmap a sparse file but I can't think of any.