Restoring each zstd-compressed full-page image currently creates and destroys a decompression context. One decompression context gets now saved in a XLogReaderState on first use, reused for the decompression of any follow-up zstd-compressed pages.

The decompression context is freed in XLogReaderFree(). In non-FRONTEND builds, a memory context reset callback is registered to make sure that the decompression context does not leak. Note that zstd has APIs to either initialize a pre-allocated decompression context or assign custom allocation and free routines, but none of these are available for builds that dynamically link to the zstd library (ZSTDLIB_STATIC_API).

Tomas Vondra has done some benchmarking showing that for a FPI-heavy set of WAL records, reusing the decompression context can nicely improve the recovery time, bringing it close to lz4. In my own measurements, I have seen up to a 4% faster recovery time when replaying records made mostly of zstd-compressed full-page images between HEAD and this improvement. Andrey has seen better numbers than that.

• Author: Andrey Borodin x4mmm@yandex-team.ru
• Reviewed-by: Michael Paquier michael@paquier.xyz
• Discussion