caching

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title: Caching permalink: /caching parent: Deep Dives nav_order: 8

The Caching Engine

SimpleZipDrive uses a hybrid, tiered caching design so that browsing feels local while keeping memory bounded.

Tier overview

flowchart LR
    R[Read request] --> S{Stored ZIP entry?}
    S -- "Yes" --> Z["Zero-copy direct read<br/>(4 MB read-ahead)<br/>no cache"]
    S -- "No" --> M{Entry size ≤<br/>per-file RAM limit?}
    M -- "Yes" --> MC["Shared memory cache<br/>decompress once, share buffer"]
    M -- "No" --> DC["Disk cache<br/>extract once to temp file"]
    MC -- "memory limit hit / OOM" --> DC

Loading

1. Zero-copy path for stored ZIP entries

A ZIP entry that is stored (not deflated), not encrypted, and not part of a solid archive is never decompressed at all. The reader opens a seekable window directly over the archive's bytes at the entry's data offset:

• Reads use System.IO.RandomAccess positional reads on the archive's file handle — no stream-position contention between concurrent readers.
• Sequential access triggers a 4 MB read-ahead buffer.
• This gives near-native throughput (measured 500+ MB/s on SSDs) with a tiny, fixed memory footprint.

2. Shared memory cache for small entries

Compressed entries that fit within the per-entry memory limit are decompressed exactly once into an exact-size buffer:

• Decompress-once, share across handles. Every process opening the file gets its own read-only stream over the same buffer, with an atomic reference count. Opening the same file 100 times costs one decompression.
• Warm buffers. When the last handle closes, the buffer stays in the cache (reference count 0) so the next open is instant. It is evicted only when memory is needed — LRU by last-use time.
• First decompression is serialized per entry (a per-entry semaphore) — parallel opens do not duplicate work.
• Exact-size buffers. Decompression writes directly into the final buffer (no intermediate MemoryStream + ToArray() copy). This halved peak memory in 2.9.0: a 424 MB entry now peaks at ≈ one copy of the data instead of two.

Limits:

Limit	Default / rule
Per-entry RAM limit	MaxMemoryPerFileMb setting, 512 MB default, clamped to 1 MB … 90 % of installed RAM
Total memory-cache budget	90 % of total available memory (GC.GetGCMemoryInfo().TotalAvailableMemoryBytes)

When an allocation would exceed the total budget — or the system throws OutOfMemoryException — the entry falls back to the disk cache instead of failing ("Memory limit approaching. Using disk cache for small file '…'").

Task Manager note: the working set after first open is roughly app base + file size for the largest file you touched. The 2.9.0 fix removed the transient double allocation that made memory appear to "double" between runs (see issue #10).

3. Disk cache for large entries

Entries larger than the per-entry limit (or decompressed when memory is exhausted) are extracted once to a file in the session temp directory:

• Location: %LOCALAPPDATA%\SimpleZipDrive\Temp\<pid>_<guid>\<n>.tmp — one directory per mount session.
• Temp files are created with a secure ACL granting full control to the current user only.
• The extracted file is registered in an in-session cache and reused by all subsequent opens ("Reusing existing temporary cache for '…'").
• Free disk space is checked first: "Insufficient disk space to extract file '…'. Available: … Required: …".
• Reads come from the extracted file via regular file I/O, so Windows' own file cache accelerates repeat access.

Cleanup

• On unmount / exit: the session's temp directory is deleted recursively; all disk-cache files are removed; the memory cache is cleared; per-entry semaphores are disposed.
• On startup (orphan sweep): directories matching <pid>_<guid> whose PID no longer exists — or whose PID was reused by a different process — are deleted. This cleans up after crashes without touching live sessions.
• Manual: the Clean Temp Files menu item runs the same sweep on demand.
• Cross-integrity mount folders (%LOCALAPPDATA%\SimpleZipDrive\Mounts\…) are not auto-deleted — empty folders may accumulate and can be removed manually.

Tuning

• Lower MaxMemoryPerFileMb (e.g. 64–128) on RAM-constrained machines — larger files then go to the disk cache instead of RAM (settings).
• Keep it high (512+ MB) for fastest repeated access to big game/media files, provided you have the RAM.
• The zero-copy stored-entry path is unaffected by these limits — it never buffers the whole file.