Merge fix for PXB-3543 from 8.0 to release-8.0.35-35 - #1724
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…al backup for 1 Million tables https://perconadev.atlassian.net/browse/PXB-3543 Optimize incremental backup buffer initialization **Problem:** Incremental backups were suffering from extremely high CPU usage (up to 82% in perf reports) attributed to `__memset_avx512_unaligned_erms`. The root cause was the initialization of the delta buffer (~67MB per thread). The kernel lazily allocates physical RAM, so the initial `memset` triggered millions of Page Faults (`asm_exc_page_fault`), forcing the kernel to pause execution, allocate RAM, and zero it out. Additionally, flushing the buffer involved a redundant `memset` that wasted memory bandwidth. **Solution:** 1. Removed the full-buffer `memset` in `wf_incremental_init`. 2. Removed the full-buffer `memset` in `wf_incremental_process` (flush loop). 3. Replaced both with a "surgical" `memset` that only zeroes the first page (the Index Page). **Safety Analysis:** The delta buffer consists of Page 0 (Index/Header) and Pages 1..N (Data). - **Page 0:** Must be zeroed to ensure the file headers are clean and no garbage data exists after the `0xFFFFFFFF` sentinel. The new `memset` handles this. - **Pages 1..N:** These are populated sequentially via `memcpy`. We track the number of valid pages (`npages`) and only write `npages * page_size` to disk. Any "dirty" or uninitialized memory at the tail of the buffer is never read and never written to the file. **Impact:** - Eliminates the "First Touch" page fault storm during initialization. - Saves ~67MB of memory write bandwidth per flush cycle per thread. - Drastically reduces system CPU time during incremental backups.
PXB-3543 [8.0] : Xtrabackup 8.0 is extremely slow when performing incremenal backup for 1 Million Tables
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