[wolf550e]

Imagine a torrent client (or http client downloading a file in parallel using HTTP range requests). It creates an empty file and then it has downloaded a 1MB of data to write at offset 100GB and wants to write it to disk. It does not want to pay the price of waiting for 100GB of zeroes to be written. The other blocks will all be downloaded and written eventually, all out of order. If the filesystem had an atomic operation to transform a bunch of (block aligned) files into a single file (like AWS S3 Multipart Upload), then sparse files would not be needed for this case.

[loeg]

Fallocate is a much better interface for this than sparse files. The torrent client does not care how the underlying filesystem provides the ability to randomly write a large file. And fallocate is a much clearer signal to the filesystem than a sparse file.

[avianlyric]

fallocate is just an interface to create sparse files. The result of using `fallocate` is a sparse file.

[loeg]

You should read my comment in the context of the one it is replying to. That comment suggested a torrent client using seeks + writes to randomly insert chunks as they were downloaded. I have summarized this approach in my comment as "sparse files," expecting charitable readers to be familiar with the context. This method of creating sparse files does not tell the filesystem anything about the intent of the application and usually creates a bunch of fragmentation under torrent-like workloads.

[avianlyric]

“sparse files” are specific term[1] referring to files where the filesystem tracks and doesn’t allocate space for unwritten file content (i.e. content that would just be zeros if read) in large preallocated files.

To use the term “sparse file” to also refer to files with large continuous runs of zeros, created via a seek operation, is just confusing. Those are quite explicitly not sparse files, they’re just files, that happen to be full of zeros (all written to disk). “Sparse file” are quite explicitly the result of the optimisation to avoid writing pointless zeros when preallocating a large file that’s going to written into in an unordered manner.

Using the term “sparse files” to refer to both the “problem” and the “solution” is just unhelpful, and doesn’t align with the accepted meaning of the term.

[1] https://en.m.wikipedia.org/wiki/Sparse_file

[epcoa]

It’s not about being charitable. For those unfamiliar with the terminology this is just confusing, and for those that are familiar this discussion is all fundamental and well known anyway.

Unfortunately for COW filesystems including zfs and btrfs fallocate doesn’t do anything useful for preallocation. You’re still going to get fragmentation. The two methods outlined are essentially equivalent.

[loeg]

> For those unfamiliar with the terminology this is just confusing, and for those that are familiar this discussion is all fundamental and well known anyway.

Eh, agree to disagree.

> Unfortunately for COW filesystems including zfs and btrfs fallocate doesn’t do anything useful for preallocation.

Both ZFS and BtrFS have "nocow" modes that are probably more suitable to this type of use case. And other filesystems are widely used.

[finnh]

Can you point me to docs for ZFS offering a nocow mode? I haven't used it in about a decade, but i can't see how that would work - wafl/cow is a pretty fundamental invariant in everything ZFS does

[myself248]

Out of curiosity, do you know off-hand how torrent clients do it on filesystems that don't support sparse files? There must be either a preallocate-the-whole-thing step, or a gather-the-pieces-together-and-write-out-the-large-file step. The latter would seem to briefly double the disk space needed at the end of the download, so I suspect they do the former.

[ajross]

Chunk it up and resassemble, one assumes. Things aren't nearly as clear in the modern world of gigabit pipes into suburban households[1], but when these things were written the filesystem was 100x faster than the link to those peer connections from which the data was fetched. A final copy was only a small overhead.

[1] Which is why all the stuff we used to torrent is in the cloud now.

[cesarb]

AFAIK, they preallocate; and even on filesystems which support sparse files, most bittorrent clients have an option to always preallocate (to both reserve space and reduce fragmentation).