Summary:
Historically, `BlobFileReader` has returned the blob(s) read from the file
in the `PinnableSlice` provided by the client. This interface was
preserved when caching was implemented for blobs, which meant that
the blob data was copied multiple times when caching was in use: first,
into the client-provided `PinnableSlice` (by `BlobFileReader::SaveValue`),
and then, into the object stored in the cache (by `BlobSource::PutBlobIntoCache`).
The patch eliminates these copies and the related allocations by changing
`BlobFileReader` so it returns its results in the form of heap-allocated `BlobContents`
objects that can be directly inserted into the cache. The allocations backing
these `BlobContents` objects are made using the blob cache's allocator if the
blobs are to be inserted into the cache (i.e. if a cache is configured and the
`fill_cache` read option is set). Note: this PR focuses on the common case when
blobs are compressed; some further small optimizations are possible for uncompressed
blobs.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/10647
Test Plan: `make check`
Reviewed By: riversand963
Differential Revision: D39335185
Pulled By: ltamasi
fbshipit-source-id: 464503d60a5520d654c8273ffb8efd5d1bcd7b36
Levi Tamasi