[pbs-devel] [PATCH proxmox-backup 0/3] datastore: remove config reload on hot path

[Samuel Rufinatscha <s.rufinatscha@proxmox.com>]

Hi,

this series reduces CPU time in datastore lookups by avoiding repeated
datastore.cfg reads/parses in both `lookup_datastore()` and
`DataStore::Drop`. It also adds a TTL so manual config edits are
noticed without reintroducing hashing on every request.

While investigating #6049 [1], cargo-flamegraph [2] showed hotspots during
repeated `/status` calls in `lookup_datastore()` and in `Drop`,
dominated by `pbs_config::datastore::config()` (config parse).

The parsing cost itself should eventually be investigated in a future
effort. Furthermore, cargo-flamegraph showed that when using a
token-based auth method to access the API, a significant amount of time
is spent in validation on every request, likely related to bcrypt.
Also this should be eventually revisited in a future effort.

## Approach

[PATCH 1/3] Fast path for datastore lookups
Use the shared-memory `ConfigVersionCache` generation for `datastore.cfg`.
Tag each cached `DataStoreImpl` with the last seen generation; when it
matches, reuse the cached instance. Fall back to the existing slow path
on mismatch or when the cache is unavailable.

[PATCH 2/3] Fast path for `Drop`
Reuse the maintenance mode eviction decision captured at lookup time,
removing the config reload from `Drop`.

[PATCH 3/3] TTL to catch manual edits
If a cached entry is older than `DATASTORE_CONFIG_CACHE_TTL_SECS`
(default 60s), the next lookup refreshes it via the slow path. This
detects manual file edits without hashing on every request.

## Results

End-to-end `/status?verbose=0` (1000 stores, 5 req/store, parallel=16):

  Metric                 Baseline   [1/3]    [2/3]
  ------------------------------------------------
  Total time             13s        11s      10s
  Throughput (all)       384.62     454.55   500.00
  Cold RPS (round #1)    76.92      90.91    100.00
  Warm RPS (2..N)        307.69     363.64   400.00

Patch 1 improves overall throughput by ~18% (−15% total time). Patch 2
adds ~10% on top. Patch 3 is a robustness feature; a 0.1 s probe shows
periodic latency spikes at TTL expiry and flat latencies otherwise.

## Reproduction steps

VM: 4 vCPU, ~8 GiB RAM, VirtIO-SCSI; disks:
  - scsi0 32G (OS)
  - scsi1 1000G (datastores)

Install PBS from ISO on the VM.

Set up ZFS on /dev/sdb (adjust if different):

    zpool create -f -o ashift=12 pbsbench /dev/sdb
    zfs set mountpoint=/pbsbench pbsbench
    zfs create pbsbench/pbs-bench

Raise file-descriptor limit:

    sudo systemctl edit proxmox-backup-proxy.service

Add the following lines:

    [Service]
    LimitNOFILE=1048576

Reload systemd and restart the proxy:

    sudo systemctl daemon-reload
    sudo systemctl restart proxmox-backup-proxy.service

Verify the limit:

    systemctl show proxmox-backup-proxy.service | grep LimitNOFILE

Create 1000 ZFS-backed datastores (as used in #6049 [1]):

    seq -w 001 1000 | xargs -n1 -P1 bash -c '
      id=$0
      name="ds${id}"
      dataset="pbsbench/pbs-bench/${name}"
      path="/pbsbench/pbs-bench/${name}"
      zfs create -o mountpoint="$path" "$dataset"
      proxmox-backup-manager datastore create "$name" "$path" \
        --comment "ZFS dataset-based datastore"
    '

Build PBS from this series, then run the server under manually
under flamegraph:

    systemctl stop proxmox-backup-proxy
    cargo flamegraph --release --bin proxmox-backup-proxy

Benchmark script (`bench.sh`) used for the numbers above:

    #!/usr/bin/env bash
    set -euo pipefail

    # --- Config ---------------------------------------------------------------
    HOST='https://localhost:8007'
    USER='root@pam'
    PASS="$(cat passfile)"

    DATASTORE_PATH="/pbsbench/pbs-bench"
    MAX_STORES=1000      # how many stores to include
    PARALLEL=16         # concurrent workers
    REPEAT=5            # requests per store (1 cold + REPEAT-1 warm)

    PRINT_FIRST=false   # true => log first request's HTTP code per store

    # --- Helpers --------------------------------------------------------------
    fmt_rps () {
      local n="$1" t="$2"
      awk -v n="$n" -v t="$t" 'BEGIN { if (t > 0) printf("%.2f\n", n/t); else print "0.00" }'
    }

    # --- Login ---------------------------------------------------------------
    auth=$(curl -ks -X POST "$HOST/api2/json/access/ticket" \
      -d "username=$USER" -d "password=$PASS")
    ticket=$(echo "$auth" | jq -r '.data.ticket')

    if [[ -z "${ticket:-}" || "$ticket" == "null" ]]; then
      echo "[ERROR] Login failed (no ticket)"
      exit 1
    fi

    # --- Collect stores (deterministic order) --------------------------------
    mapfile -t STORES < <(
      find "$DATASTORE_PATH" -mindepth 1 -maxdepth 1 -type d -printf '%f\n' \
      | sort | head -n "$MAX_STORES"
    )

    USED_STORES=${#STORES[@]}
    if (( USED_STORES == 0 )); then
      echo "[ERROR] No datastore dirs under $DATASTORE_PATH"
      exit 1
    fi

    echo "[INFO] Running with stores=$USED_STORES, repeat=$REPEAT, parallel=$PARALLEL"

    # --- Temp counters --------------------------------------------------------
    SUCCESS_ALL="$(mktemp)"
    FAIL_ALL="$(mktemp)"
    COLD_OK="$(mktemp)"
    WARM_OK="$(mktemp)"
    trap 'rm -f "$SUCCESS_ALL" "$FAIL_ALL" "$COLD_OK" "$WARM_OK"' EXIT

    export HOST ticket REPEAT SUCCESS_ALL FAIL_ALL COLD_OK WARM_OK PRINT_FIRST

    SECONDS=0

    # --- Fire requests --------------------------------------------------------
    printf "%s\n" "${STORES[@]}" \
    | xargs -P"$PARALLEL" -I{} bash -c '
      store="$1"
      url="$HOST/api2/json/admin/datastore/$store/status?verbose=0"

      for ((i=1;i<=REPEAT;i++)); do
        code=$(curl -ks -o /dev/null -w "%{http_code}" -b "PBSAuthCookie=$ticket" "$url" || echo 000)

        if [[ "$code" == "200" ]]; then
          echo 1 >> "$SUCCESS_ALL"
          if (( i == 1 )); then
            echo 1 >> "$COLD_OK"
          else
            echo 1 >> "$WARM_OK"
          fi
          if [[ "$PRINT_FIRST" == "true" && $i -eq 1 ]]; then
            ts=$(date +%H:%M:%S)
            echo "[$ts] $store #$i HTTP:200"
          fi
        else
          echo 1 >> "$FAIL_ALL"
          if [[ "$PRINT_FIRST" == "true" && $i -eq 1 ]]; then
            ts=$(date +%H:%M:%S)
            echo "[$ts] $store #$i HTTP:$code (FAIL)"
          fi
        fi
      done
    ' _ {}

    # --- Summary --------------------------------------------------------------
    elapsed=$SECONDS
    ok=$(wc -l < "$SUCCESS_ALL" 2>/dev/null || echo 0)
    fail=$(wc -l < "$FAIL_ALL" 2>/dev/null || echo 0)
    cold_ok=$(wc -l < "$COLD_OK" 2>/dev/null || echo 0)
    warm_ok=$(wc -l < "$WARM_OK" 2>/dev/null || echo 0)

    expected=$(( USED_STORES * REPEAT ))
    total=$(( ok + fail ))

    rps_all=$(fmt_rps "$ok" "$elapsed")
    rps_cold=$(fmt_rps "$cold_ok" "$elapsed")
    rps_warm=$(fmt_rps "$warm_ok" "$elapsed")

    echo "===== Summary ====="
    echo "Stores used:         $USED_STORES"
    echo "Expected requests:   $expected"
    echo "Executed requests:   $total"
    echo "OK (HTTP 200):       $ok"
    echo "Failed:              $fail"
    printf "Total time:          %dm %ds\n" $((elapsed/60)) $((elapsed%60))
    echo "Throughput all RPS:  $rps_all"
    echo "Cold RPS (round #1): $rps_cold"
    echo "Warm RPS (#2..N):    $rps_warm"

## Maintainer notes

- No dependency bumps, no API changes, no breaking changes in this
  series.

## Patch summary

[PATCH 1/3] partial fix #6049: datastore: impl ConfigVersionCache fast path for lookups
[PATCH 2/3] partial fix #6049: datastore: use config fast-path in Drop
[PATCH 3/3] datastore: add TTL fallback to catch manual config edits

Thanks for reviewing!

[1] Bugzilla #6049: https://bugzilla.proxmox.com/show_bug.cgi?id=6049
[2] cargo-flamegraph: https://github.com/flamegraph-rs/flamegraph

Samuel Rufinatscha (3):
  partial fix #6049: datastore: impl ConfigVersionCache fast path for
    lookups
  partial fix #6049: datastore: use config fast-path in Drop
  datastore: add TTL fallback to catch manual config edits

 pbs-config/src/config_version_cache.rs |  10 ++-
 pbs-datastore/src/datastore.rs         | 119 ++++++++++++++++++-------
 2 files changed, 96 insertions(+), 33 deletions(-)

-- 
2.47.3



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