[PERF] Memory-mapped SSTable reads — zero-copy I/O via mmap for cold data

[ElioNeto]

Description

ApexStore currently reads SSTable data through SstableReader with explicit read() syscalls. MMAP allows the OS to manage disk-to-memory paging transparently, reducing syscall overhead and enabling better cache utilization.

Proposed Implementation

1. Add mmap_read: bool to StorageConfig (default: false for compatibility)
2. When enabled, open SSTable files with memmap2 crate
3. Serve read requests directly from the mmap region (zero-copy)
4. Use MADV_RANDOM or MADV_SEQUENTIAL advice depending on access pattern
5. Fall back to pread() for blocks that fail to mmap (e.g., file too large on 32-bit)

Expected Impact

• 20-40% reduction in cold read latency (fewer syscalls)
• Reduced memory pressure (OS manages page cache vs application buffer)
• Faster database restarts (no need to pre-warm cache)

Labels

[ElioNeto]

Fixed in commit 1d8c830.