> We initially considered limiting transcoding to popular content
Weird, I would have compressed cold content instead, if the goal was to save on CPU time during decode.
genxy 2 hours ago [-]
I would compress it all, and then selectively recompress at higher compression levels depending on the link, read frequency, diversity and capabilities of the clients.
Zstd 3 to 5 is nearly free in terms of not bottlenecking disk or network. Zstd 12 to 19 gives amazing compression results and still result in speedups when reading from disk. It really is a wonderful all purpose compressor.
One of the nice things about Zstd is if you try to compress an already compressed stream, it short circuits. So even if you are given say HVEC MP4 and run zstd -19 on it, it will "compress" immediately and not DOS your pipeline.
mort96 44 minutes ago [-]
It has mostly become my go-to as well. Just wish it wasn't a Facebook product.
articulatepang 30 minutes ago [-]
I agree! I came to the comment section to say exactly this. In any cache hierarchy you want to put colder content in cheaper but slower storage. Here, compression is the cheaper but slower form of storage.
CodesInChaos 2 hours ago [-]
I'm confused by how this affects range requests. Without compression, those can be easily satisfied by reading the relevant part of the cached complete file. But how are they handled now? The article claims "range requests remain unchanged", but I don't see how that's possible if the cache no longer stores the uncompressed data.
nijave 4 minutes ago [-]
Idk but btrfs and zfs manage to pull it off
Seekable OCI (SOCI) uses an index so I imagine that's an option (real byte range a-b maps to compressed range x-y). Presumably you'd still need to read the header and some additional pieces
gopalv 17 minutes ago [-]
> I don't see how that's possible if the cache no longer stores the uncompressed data.
Zstd has a seekable format for frames, similar to pigz --independent works.
I assume the entire resource needs to be decompressed first, then indexed into, served, and discarded. Well, actually, you could just decompress up to the end of the range.
kccqzy 29 minutes ago [-]
Actually zstd internally splits data into frames, and frames can indicate the decompressed data size. So if we control the compressor we can make it so that all frames have the size information; it isn’t exactly seekable but at least it will not need to decompress the resource. https://python-zstandard.readthedocs.io/en/latest/concepts.h...
Given how fast zstd can decompress, this might not actually be a win.
CodesInChaos 1 hours ago [-]
Which would have terrible performance for range requests starting late in a large file. For files that are frequently accessed that way, this could be prohibitive.
You could split the file into independently compressed blocks as well. But that'd reduce compression rate and require adding some kind of index for seeking.
Or they have an upper size limit for the file size they compress, since large files are rarely compressible text.
In any case it is something that needs the be handled before going live with a compressed cache. But the article sounds like they simply didn't implement compressed caching for those cases, which makes no sense.
genxy 1 hours ago [-]
Not with zstd, you could still support range requests. https://en.wikipedia.org/wiki/Zstd this whole subthread should take 10 minutes and glance over the spec and the capabilities. It would end a lot of wasted premature pontificating.
https://caniuse.com/?search=zstd
Weird, I would have compressed cold content instead, if the goal was to save on CPU time during decode.
Zstd 3 to 5 is nearly free in terms of not bottlenecking disk or network. Zstd 12 to 19 gives amazing compression results and still result in speedups when reading from disk. It really is a wonderful all purpose compressor.
One of the nice things about Zstd is if you try to compress an already compressed stream, it short circuits. So even if you are given say HVEC MP4 and run zstd -19 on it, it will "compress" immediately and not DOS your pipeline.
Seekable OCI (SOCI) uses an index so I imagine that's an option (real byte range a-b maps to compressed range x-y). Presumably you'd still need to read the header and some additional pieces
Zstd has a seekable format for frames, similar to pigz --independent works.
[1] - https://github.com/facebook/zstd/blob/dev/contrib/seekable_f...
Given how fast zstd can decompress, this might not actually be a win.
You could split the file into independently compressed blocks as well. But that'd reduce compression rate and require adding some kind of index for seeking.
Or they have an upper size limit for the file size they compress, since large files are rarely compressible text.
In any case it is something that needs the be handled before going live with a compressed cache. But the article sounds like they simply didn't implement compressed caching for those cases, which makes no sense.
There's this, but it doesn't seem to be getting much traction: https://github.com/facebook/zstd/tree/dev/contrib/seekable_f...