forked from wrenn/wrenn
pptx704
01819642cc
Linux keeps freed memory as page cache, which Firecracker snapshots as non-zero blocks. A 16GB VM with 12GB stale cache would write all 12GB to disk. Dropping pagecache (not dentries/inodes) in /snapshot/prepare before blocking the reclaimer shrinks snapshots to actual working set size with minimal resume latency impact.
50 lines
1.7 KiB
Rust
50 lines
1.7 KiB
Rust
use std::sync::Arc;
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use std::sync::atomic::Ordering;
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use axum::extract::State;
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use axum::http::{StatusCode, header};
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use axum::response::IntoResponse;
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use crate::state::AppState;
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/// POST /snapshot/prepare — quiesce subsystems before Firecracker snapshot.
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///
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/// In Rust there is no GC dance. We just:
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/// 1. Drop page cache to shrink snapshot size
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/// 2. Stop port subsystem
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/// 3. Close idle connections via conntracker
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/// 4. Set needs_restore flag
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pub async fn post_snapshot_prepare(State(state): State<Arc<AppState>>) -> impl IntoResponse {
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// Drop page cache BEFORE blocking the reclaimer — avoids snapshotting
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// gigabytes of stale cache that inflates the memory dump on disk.
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// "1" = pagecache only (keep dentries/inodes for faster resume).
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if let Err(e) = std::fs::write("/proc/sys/vm/drop_caches", "1") {
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tracing::warn!(error = %e, "snapshot/prepare: drop_caches failed");
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} else {
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tracing::info!("snapshot/prepare: page cache dropped");
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}
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// Block memory reclaimer — prevents drop_caches from running mid-freeze
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// which would corrupt kernel page table state.
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state.snapshot_in_progress.store(true, Ordering::Release);
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if let Some(ref ps) = state.port_subsystem {
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ps.stop();
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tracing::info!("snapshot/prepare: port subsystem stopped");
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}
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state.conn_tracker.prepare_for_snapshot();
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tracing::info!("snapshot/prepare: connections prepared");
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// Sync filesystem buffers so dirty pages are flushed before freeze.
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unsafe { libc::sync(); }
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state.needs_restore.store(true, Ordering::Release);
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tracing::info!("snapshot/prepare: ready for freeze");
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(
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StatusCode::NO_CONTENT,
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[(header::CACHE_CONTROL, "no-store")],
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)
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}
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