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fix: drop page cache before snapshot to reduce memory dump size

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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.
This commit is contained in:
Rafeed M. Bhuiyan
2026-05-03 14:27:49 +06:00
parent cb28f7759d
commit 01819642cc
1 changed files with 15 additions and 5 deletions
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20
envd-rs/src/http/snapshot.rs
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@ -10,12 +10,22 @@ use crate::state::AppState;
/// POST /snapshot/prepare — quiesce subsystems before Firecracker snapshot.
///
/// In Rust there is no GC dance. We just:
/// 1. Stop port subsystem
/// 2. Close idle connections via conntracker
/// 3. Set needs_restore flag
/// 1. Drop page cache to shrink snapshot size
/// 2. Stop port subsystem
/// 3. Close idle connections via conntracker
/// 4. Set needs_restore flag
pub async fn post_snapshot_prepare(State(state): State<Arc<AppState>>) -> impl IntoResponse {
// Block memory reclaimer before anything else — prevents drop_caches
// from running mid-freeze which would corrupt kernel page table state.
// Drop page cache BEFORE blocking the reclaimer — avoids snapshotting
// gigabytes of stale cache that inflates the memory dump on disk.
// "1" = pagecache only (keep dentries/inodes for faster resume).
if let Err(e) = std::fs::write("/proc/sys/vm/drop_caches", "1") {
tracing::warn!(error = %e, "snapshot/prepare: drop_caches failed");
} else {
tracing::info!("snapshot/prepare: page cache dropped");
}
// Block memory reclaimer — prevents drop_caches from running mid-freeze
// which would corrupt kernel page table state.
state.snapshot_in_progress.store(true, Ordering::Release);
if let Some(ref ps) = state.port_subsystem {