4 ms·

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 o

by pkulak 1mo ago

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.

CodesInChaos 1mo 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.
- mgerdts 1mo ago
  For the cost of a small amount of metadata the offsets of every MiB or so could be stored. I did something like this with pigz as I was implementing multithreaded compressed and encrypted kernel zone suspend and resume for Solaris.
genxy 1mo ago
Not with zstd, you could still support range requests. https://en.wikipedia.org/wiki/Zstd 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.
- ncruces 1mo ago
  There's nothing about random access at that link. There's this, but it doesn't seem to be getting much traction: https://github.com/facebook/zstd/tree/dev/contrib/seekable_format https://github.com/facebook/zstd/tree/dev/contrib/seekable_f...
  - genxy 1mo ago
    Also external dictionaries https://nigeltao.github.io/blog/2022/zstandard-part-7-dictionaries.html https://nigeltao.github.io/blog/2022/zstandard-part-7-dictio...
kccqzy 1mo 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.html https://python-zstandard.readthedocs.io/en/latest/concepts.h... Given how fast zstd can decompress, this may or may not actually be a win: the time spent waiting for I/O might be so large that the decompression can fit within the wait time.

by pkulak 1mo ago

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.

CodesInChaos 1mo 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.

mgerdts 1mo ago
For the cost of a small amount of metadata the offsets of every MiB or so could be stored. I did something like this with pigz as I was implementing multithreaded compressed and encrypted kernel zone suspend and resume for Solaris.

genxy 1mo ago

Not with zstd, you could still support range requests. https://en.wikipedia.org/wiki/Zstd 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.

ncruces 1mo ago
There's nothing about random access at that link. There's this, but it doesn't seem to be getting much traction: https://github.com/facebook/zstd/tree/dev/contrib/seekable_format https://github.com/facebook/zstd/tree/dev/contrib/seekable_f...
- genxy 1mo ago
  Also external dictionaries https://nigeltao.github.io/blog/2022/zstandard-part-7-dictionaries.html https://nigeltao.github.io/blog/2022/zstandard-part-7-dictio...

kccqzy 1mo 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.html https://python-zstandard.readthedocs.io/en/latest/concepts.h... Given how fast zstd can decompress, this may or may not actually be a win: the time spent waiting for I/O might be so large that the decompression can fit within the wait time.