gunnar_server/

disktest.rs

//! **The start-up disk test**: what /data can actually do, said once on the
//! screen.
//!
//! ```text
//!  17:40:43  disktest /data  seq w 812 MB/s · r 1.9 GB/s · 4k sync 3100 ops/s  (1.6 s)
//! ```
//!
//! Three numbers, because they are the three a git server lives on: a
//! sequential write (a pack arriving), a sequential read (a clone leaving),
//! and 4 KiB overwrites each followed by `fsync` — the loose-object / ref
//! update pattern, and the number that separates a FAT /data from an xfs one
//! (an open item; the rig reads this row).
//!
//! The write and the 4 KiB legs are plain `std::fs`. The READ leg measures the
//! device, not the page cache: the file it just wrote is still cached, so the
//! read reopens it with `O_DIRECT` (one 4 MiB buffer, 4 KiB-aligned,
//! allocated once for the whole run) and streams it back off the disk. Where
//! `O_DIRECT` is refused (tmpfs, some FUSE) the fallback is
//! `posix_fadvise(DONTNEED)` to drop the cached pages, then a buffered read —
//! and a result that is still implausible (over 5 GB/s) is said to be
//! `cached` rather than reported as disk speed. On the appliance `/data` is
//! the mounted xfs over the whole md raid0 set, so the numbers are the
//! stripe's, not one member disk's. 256 MiB under `<data>/.korp-disktest`,
//! deleted afterwards. Runs on its own thread the moment `/data` is known to
//! be mounted and never delays serving: the row arrives when it arrives.
//!
//! Never a panic, never a reason PID 1 is unhappy: too little free space is a
//! `skipped` row, any I/O error is one WARN row, and both leave the file gone.

use std::fs::{File, OpenOptions};
use std::io::{Read, Seek, SeekFrom, Write};
use std::os::unix::fs::OpenOptionsExt;
use std::path::{Path, PathBuf};
use std::time::{Duration, Instant};

use rustix::fs::{fadvise, Advice, OFlags};

/// The env seam: `GUNNAR_DISKTEST=1` runs it on a developer box regardless
/// of the PID-1 gate.
pub const ENV: &str = "GUNNAR_DISKTEST";
/// The file's directory under the data root; removed when the test is done.
pub const DIR_NAME: &str = ".korp-disktest";
/// What the test writes.
pub const FILE_BYTES: u64 = 256 << 20;
/// Skip below this much free space: the test must never be the thing that
/// fills the disk.
pub const MIN_FREE_BYTES: u64 = 1 << 30;
/// How long the 4 KiB fsync loop runs.
pub const SYNC_WINDOW: Duration = Duration::from_millis(500);
/// A read faster than this did not touch the disk.
pub const CACHED_BPS: f64 = 5e9;
/// The write chunk and the read buffer: one buffer of this size, allocated
/// once per run, aligned for `O_DIRECT`.
const CHUNK: usize = 4 << 20;
const BLOCK: usize = 4096;
/// `O_DIRECT` wants the buffer, the offset and the length aligned to the
/// logical block; 4 KiB satisfies every disk the appliance runs on.
const DIRECT_ALIGN: usize = 4096;

/// How the read leg got its bytes.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ReadHow {
    /// `O_DIRECT`: the page cache was bypassed, the number is the device's.
    Direct,
    /// `O_DIRECT` refused; the cached pages were dropped (`fadvise
    /// DONTNEED`) and the file read through the cache.
    Dropped,
}

impl ReadHow {
    pub fn as_str(self) -> &'static str {
        match self {
            ReadHow::Direct => "direct",
            ReadHow::Dropped => "dropped",
        }
    }
}

/// What the test measured.
#[derive(Debug, Clone, PartialEq)]
pub struct Report {
    pub seq_write_bps: f64,
    pub seq_read_bps: f64,
    /// Whether the read bypassed the page cache or only dropped it first.
    pub read_how: ReadHow,
    pub sync_ops_per_s: f64,
    pub elapsed: Duration,
    /// **What the rates are made of** (2026-09-28): the bytes written and
    /// read back, each leg's own clock, and the fsync'd 4 KiB operations the
    /// sync rate counts — so a reader can check a rate against the run that
    /// produced it instead of taking a derived number on trust.
    pub file_bytes: u64,
    pub write_elapsed: Duration,
    pub read_elapsed: Duration,
    pub sync_ops: u64,
    pub sync_elapsed: Duration,
}

impl Report {
    /// `seq w 812 MB/s · r 1.9 GB/s · 4k sync 3100 ops/s  (1.6 s)`; a read
    /// over [`CACHED_BPS`] reads `r cached`.
    pub fn row(&self) -> String {
        let read = if self.seq_read_bps > CACHED_BPS {
            "cached".to_owned()
        } else {
            fmt_rate(self.seq_read_bps)
        };
        format!(
            "seq w {} \u{b7} r {} \u{b7} 4k sync {:.0} ops/s  ({:.1} s)",
            fmt_rate(self.seq_write_bps),
            read,
            self.sync_ops_per_s,
            self.elapsed.as_secs_f64()
        )
    }
}

/// `812 MB/s` / `1.9 GB/s` (decimal units, as disks are sold).
pub fn fmt_rate(bps: f64) -> String {
    if bps >= 1e9 {
        format!("{:.1} GB/s", bps / 1e9)
    } else if bps >= 1e6 {
        format!("{:.0} MB/s", bps / 1e6)
    } else {
        format!("{:.0} kB/s", bps / 1e3)
    }
}

/// How one run ended.
#[derive(Debug, Clone, PartialEq)]
pub enum Outcome {
    Done(Report),
    /// Too little free space: `(free, needed)` bytes.
    Skipped {
        free: u64,
        need: u64,
    },
    Failed(String),
}

impl Outcome {
    /// The row's text; the kind is `disktest` either way, `Failed` warns.
    pub fn row(&self) -> (String, bool) {
        match self {
            Outcome::Done(r) => (r.row(), false),
            Outcome::Skipped { free, need } => (
                format!(
                    "skipped: {} free (< {})",
                    crate::dashboard::fmt_bytes(*free),
                    crate::dashboard::fmt_bytes(*need)
                ),
                false,
            ),
            Outcome::Failed(why) => (format!("failed: {why}"), true),
        }
    }
}

/// The skip rule, pure: `free` bytes available against [`MIN_FREE_BYTES`]
/// (or `need`).
pub fn should_skip(free: u64, need: u64) -> bool {
    free < need
}

/// Run the test under `data_root` with the production sizes.
pub fn run(data_root: &Path) -> Outcome {
    run_sized(data_root, FILE_BYTES, MIN_FREE_BYTES, SYNC_WINDOW)
}

/// The test at any size — what the unit test calls with a few MiB.
pub fn run_sized(
    data_root: &Path,
    file_bytes: u64,
    min_free: u64,
    sync_window: Duration,
) -> Outcome {
    let free = match crate::dashboard::probe::disk(data_root) {
        Some((_, avail)) => avail,
        None => {
            return Outcome::Failed(format!(
                "statfs {}: not a mounted filesystem",
                data_root.display()
            ))
        }
    };
    if should_skip(free, min_free) {
        return Outcome::Skipped {
            free,
            need: min_free,
        };
    }
    let dir = data_root.join(DIR_NAME);
    let path = dir.join("probe");
    let result = measure(&dir, &path, file_bytes, sync_window);
    // Leave nothing behind whichever way it went.
    let _ = std::fs::remove_file(&path);
    let _ = std::fs::remove_dir(&dir);
    match result {
        Ok(report) => Outcome::Done(report),
        Err(why) => Outcome::Failed(why),
    }
}

fn measure(
    dir: &Path,
    path: &Path,
    file_bytes: u64,
    sync_window: Duration,
) -> Result<Report, String> {
    let started = Instant::now();
    std::fs::create_dir_all(dir).map_err(|e| format!("mkdir {}: {e}", dir.display()))?;
    // Not zeros: a filesystem or a virtual disk that compresses or
    // deduplicates would report a write it never made. ONE buffer for the
    // whole run — the write source, the O_DIRECT read target and the 4 KiB
    // block all live in it (allocate once, reuse).
    let mut buf = AlignedBuf::new(CHUNK, DIRECT_ALIGN);
    buf.fill(&pattern(CHUNK));

    // ── sequential write + fsync ───────────────────────────────────────────
    let t = Instant::now();
    let mut f = File::create(path).map_err(|e| format!("create: {e}"))?;
    let mut left = file_bytes;
    while left > 0 {
        let n = (left as usize).min(CHUNK);
        f.write_all(&buf.as_slice()[..n])
            .map_err(|e| format!("write: {e}"))?;
        left -= n as u64;
    }
    f.sync_all().map_err(|e| format!("fsync: {e}"))?;
    drop(f);
    let write_elapsed = t.elapsed();
    let seq_write_bps = file_bytes as f64 / write_elapsed.as_secs_f64().max(1e-9);

    // ── sequential read, off the device ───────────────────────────────────
    let (got, secs, read_how) = read_back(path, &mut buf, true)?;
    if got != file_bytes {
        return Err(format!("read back {got} of {file_bytes} bytes"));
    }
    let seq_read_bps = got as f64 / secs.max(1e-9);

    // ── 4 KiB random overwrites, fsync each ────────────────────────────────
    // The read leg may have filled the buffer with what it read — the same
    // pattern, but restore the block so the leg does not depend on that.
    let block_src = pattern(BLOCK);
    buf.as_mut_slice()[..BLOCK].copy_from_slice(&block_src);
    let mut f = OpenOptions::new()
        .write(true)
        .open(path)
        .map_err(|e| format!("reopen: {e}"))?;
    let blocks = (file_bytes / BLOCK as u64).max(1);
    let block = &buf.as_slice()[..BLOCK];
    let mut rng = started.elapsed().as_nanos() as u64 | 1;
    let mut ops = 0u64;
    let t = Instant::now();
    while t.elapsed() < sync_window {
        // xorshift64: enough randomness to defeat any write-combining.
        rng ^= rng << 13;
        rng ^= rng >> 7;
        rng ^= rng << 17;
        let at = (rng % blocks) * BLOCK as u64;
        f.seek(SeekFrom::Start(at))
            .map_err(|e| format!("seek: {e}"))?;
        f.write_all(block).map_err(|e| format!("write 4k: {e}"))?;
        f.sync_data().map_err(|e| format!("fdatasync: {e}"))?;
        ops += 1;
    }
    let sync_elapsed = t.elapsed();
    let sync_ops_per_s = ops as f64 / sync_elapsed.as_secs_f64().max(1e-9);
    drop(f);

    Ok(Report {
        seq_write_bps,
        seq_read_bps,
        read_how,
        sync_ops_per_s,
        elapsed: started.elapsed(),
        file_bytes,
        write_elapsed,
        read_elapsed: Duration::from_secs_f64(secs.max(0.0)),
        sync_ops: ops,
        sync_elapsed,
    })
}

/// Read `path` back sequentially into `buf`, past the page cache: `O_DIRECT`
/// when `try_direct` and the filesystem takes it, else drop the cached pages
/// (`fadvise DONTNEED`, after the caller's fsync they are clean) and read
/// buffered. Returns `(bytes, seconds, how)`; the clock starts after the
/// open so the number is the transfer, not the open.
pub fn read_back(
    path: &Path,
    buf: &mut AlignedBuf,
    try_direct: bool,
) -> Result<(u64, f64, ReadHow), String> {
    let direct = if try_direct {
        OpenOptions::new()
            .read(true)
            .custom_flags(OFlags::DIRECT.bits() as i32)
            .open(path)
            .ok()
    } else {
        None
    };
    let (mut f, how) = match direct {
        Some(f) => (f, ReadHow::Direct),
        None => {
            let f = File::open(path).map_err(|e| format!("open: {e}"))?;
            // Best effort: a filesystem that ignores the advice is one whose
            // "read" is memory anyway, and the > 5 GB/s rule catches it.
            let _ = fadvise(&f, 0, None, Advice::DontNeed);
            (f, ReadHow::Dropped)
        }
    };
    let t = Instant::now();
    let mut got = 0u64;
    loop {
        let n = f
            .read(buf.as_mut_slice())
            .map_err(|e| format!("read ({}): {e}", how.as_str()))?;
        if n == 0 {
            break;
        }
        got += n as u64;
    }
    Ok((got, t.elapsed().as_secs_f64(), how))
}

/// A heap buffer whose start is aligned to `align` (a power of two): what
/// `O_DIRECT` needs and `Vec<u8>` cannot promise. Allocated once per run.
pub struct AlignedBuf {
    raw: Vec<u8>,
    at: usize,
    len: usize,
}

impl AlignedBuf {
    pub fn new(len: usize, align: usize) -> Self {
        debug_assert!(align.is_power_of_two());
        let raw = vec![0u8; len + align];
        let at = raw.as_ptr().align_offset(align);
        Self { raw, at, len }
    }
    pub fn fill(&mut self, src: &[u8]) {
        let n = src.len().min(self.len);
        self.as_mut_slice()[..n].copy_from_slice(&src[..n]);
    }
    pub fn as_slice(&self) -> &[u8] {
        &self.raw[self.at..self.at + self.len]
    }
    pub fn as_mut_slice(&mut self) -> &mut [u8] {
        &mut self.raw[self.at..self.at + self.len]
    }
}

fn pattern(n: usize) -> Vec<u8> {
    let mut v = Vec::with_capacity(n);
    let mut x = 0x9E37_79B9_7F4A_7C15u64;
    while v.len() < n {
        x ^= x << 13;
        x ^= x >> 7;
        x ^= x << 17;
        v.extend_from_slice(&x.to_le_bytes());
    }
    v.truncate(n);
    v
}

/// **The latch.** Until 2026-09-21 the outcome went to the screen and nowhere
/// else, so nothing but a human reading a console could learn what this box's
/// disk does. The twin election compares two boxes' disks over gRPC, so the
/// last outcome is kept here and read by [`crate::capacity::MachineFacts`].
///
/// `None` means the test has not finished — the gate was closed, or the
/// thread is still running. It is never a zero and never an empty `Report`:
/// `Admin.Capacity` renders it `DISKTEST_NOT_RUN`, which is a different fact
/// from a disk that measured badly.
static LAST: std::sync::OnceLock<std::sync::Mutex<Option<Outcome>>> = std::sync::OnceLock::new();

fn latch() -> &'static std::sync::Mutex<Option<Outcome>> {
    LAST.get_or_init(|| std::sync::Mutex::new(None))
}

/// Record the outcome of a run. Called once per run, beside [`announce`].
pub fn record(outcome: &Outcome) {
    if let Ok(mut slot) = latch().lock() {
        *slot = Some(outcome.clone());
    }
}

/// The last outcome this process measured, or `None` if it has not finished
/// one. A lookup, never a run: asking this question must never start a disk
/// test, because the answer a caller wants is what the disk did at start-up.
pub fn last() -> Option<Outcome> {
    latch().lock().ok().and_then(|slot| slot.clone())
}

/// Say the outcome as the ONE tracing line the log tap turns into the
/// `disktest` row (`dashboard::describe`).
pub fn announce(data_root: &Path, outcome: &Outcome) {
    let (text, warn) = outcome.row();
    let read = match outcome {
        Outcome::Done(r) => r.read_how.as_str(),
        _ => "-",
    };
    if warn {
        tracing::warn!(event = "disktest", path = %data_root.display(), read, "{text}");
    } else {
        tracing::info!(event = "disktest", path = %data_root.display(), read, "{text}");
    }
}

/// Start the test on its own thread when this is the appliance's PID 1 (the
/// dashboard's gate) or [`ENV`] is set; say nothing otherwise. Returns
/// whether it started. Serving never waits for it.
pub fn spawn(data_root: PathBuf) -> bool {
    let explicit = std::env::var(ENV)
        .ok()
        .filter(|v| !v.is_empty() && v != "0");
    let gated = appliance_pid1::net::is_init() && appliance_pid1::net::booted_by_korp_start();
    let started = std::time::Instant::now();
    if explicit.is_none() && !gated {
        // No disk test, so nothing competes with the store for the disk and
        // the server is ready from its first bound socket. No window is opened
        // and none has to be closed.
        return false;
    }
    // **The boot window opens here.** See `crate::readiness`.
    crate::readiness::begin("the start-up disk test");
    let spawned = std::thread::Builder::new()
        .name("gunnar-disktest".into())
        .spawn(move || {
            let outcome = std::panic::catch_unwind(|| run(&data_root))
                .unwrap_or_else(|_| Outcome::Failed("the disk test panicked".to_owned()));
            record(&outcome);
            announce(&data_root, &outcome);
            // **The boot window closes here**, and only here. For the whole of
            // this thread's life another thread of this process has been
            // saturating the same disk a repository's first post-crash open
            // has to read (`4k sync 4517 ops/s (0.7 s)` on the rig), which is
            // the measured 0.6-0.7 s of `the repository is unavailable`.
            crate::readiness::mark_ready("the start-up disk test", started);
        })
        .is_ok();
    if !spawned {
        // A thread that could not be spawned must not leave the server refusing
        // every transfer for ever. The failure is already the caller's `false`.
        crate::readiness::mark_ready("a disk test that could not be spawned", started);
    }
    spawned
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn the_row_reads_as_one_line_and_a_page_cache_read_is_called_cached() {
        let r = Report {
            seq_write_bps: 812e6,
            seq_read_bps: 1.9e9,
            read_how: ReadHow::Direct,
            sync_ops_per_s: 3100.4,
            elapsed: Duration::from_millis(1600),
            file_bytes: 256 << 20,
            write_elapsed: Duration::from_millis(330),
            read_elapsed: Duration::from_millis(141),
            sync_ops: 1550,
            sync_elapsed: Duration::from_millis(500),
        };
        assert_eq!(
            r.row(),
            "seq w 812 MB/s \u{b7} r 1.9 GB/s \u{b7} 4k sync 3100 ops/s  (1.6 s)"
        );
        let cached = Report {
            seq_read_bps: 7.2e9,
            ..r.clone()
        };
        assert_eq!(
            cached.row(),
            "seq w 812 MB/s \u{b7} r cached \u{b7} 4k sync 3100 ops/s  (1.6 s)"
        );
        assert_eq!(fmt_rate(512e3), "512 kB/s");
        assert!(!Outcome::Done(r).row().1);
        assert_eq!(
            Outcome::Skipped {
                free: 800 << 20,
                need: 1 << 30
            }
            .row(),
            ("skipped: 800M free (< 1.0G)".to_owned(), false)
        );
        assert_eq!(
            Outcome::Failed("write: No space left on device".into()).row(),
            ("failed: write: No space left on device".to_owned(), true)
        );
    }

    #[test]
    fn the_skip_rule_is_free_space_below_the_need() {
        assert!(should_skip((1 << 30) - 1, 1 << 30));
        assert!(!should_skip(1 << 30, 1 << 30));
        assert!(!should_skip(u64::MAX, MIN_FREE_BYTES));
        // On a filesystem with next to nothing free the run says skipped and
        // touches no file: asking for more than any disk has.
        let dir = tempfile::tempdir().unwrap();
        let out = run_sized(dir.path(), 4 << 20, u64::MAX, Duration::from_millis(1));
        assert!(
            matches!(out, Outcome::Skipped { need: u64::MAX, .. }),
            "{out:?}"
        );
        assert!(!dir.path().join(DIR_NAME).exists());
    }

    #[test]
    fn a_small_run_measures_three_numbers_and_leaves_nothing_behind() {
        let dir = tempfile::tempdir().unwrap();
        let out = run_sized(dir.path(), 4 << 20, 0, Duration::from_millis(50));
        let Outcome::Done(r) = out else {
            panic!("{out:?}")
        };
        assert!(
            r.seq_write_bps > 0.0 && r.seq_read_bps > 0.0 && r.sync_ops_per_s > 0.0,
            "{r:?}"
        );
        assert!(r.elapsed >= Duration::from_millis(50));
        assert!(
            !dir.path().join(DIR_NAME).exists(),
            "the probe directory is removed"
        );
        let text = r.row();
        assert!(
            text.starts_with("seq w ") && text.contains(" 4k sync "),
            "{text}"
        );
    }

    /// A fixture file the read leg is pointed at: 4 MiB + a tail that is
    /// not a whole block, so the O_DIRECT short read at EOF is exercised.
    fn fixture(dir: &Path) -> (PathBuf, u64) {
        let path = dir.join("fixture");
        let body = pattern(4 * (1 << 20) + 1234);
        std::fs::write(&path, &body).unwrap();
        File::open(&path).unwrap().sync_all().unwrap();
        (path, body.len() as u64)
    }

    #[test]
    fn the_read_leg_reads_the_fixture_back_off_the_device_with_o_direct() {
        // A scratch directory on the real disk; `/tmp` is often tmpfs, which
        // refuses O_DIRECT — pick a real filesystem when one is at hand.
        let base = std::env::var_os("GUNNAR_DISKTEST_FIXTURE_DIR").map(PathBuf::from);
        let dir = match base {
            Some(b) => tempfile::tempdir_in(b).unwrap(),
            None => tempfile::tempdir().unwrap(),
        };
        let (path, len) = fixture(dir.path());
        let mut buf = AlignedBuf::new(CHUNK, DIRECT_ALIGN);
        assert_eq!(
            buf.as_slice().as_ptr() as usize % DIRECT_ALIGN,
            0,
            "the buffer is 4 KiB-aligned"
        );
        assert_eq!(buf.as_slice().len(), CHUNK);
        let (got, secs, how) = read_back(&path, &mut buf, true).unwrap();
        assert_eq!(got, len, "every byte, including the partial last block");
        assert!(secs > 0.0);
        // The bytes that landed in the buffer are the file's tail (the last,
        // short read) — the leg really read, it did not just count.
        let tail = &pattern(len as usize)[(len as usize) - 1234..];
        assert_eq!(&buf.as_slice()[..1234], tail);
        let fs_direct = OpenOptions::new()
            .read(true)
            .custom_flags(OFlags::DIRECT.bits() as i32)
            .open(&path)
            .is_ok();
        assert_eq!(
            how,
            if fs_direct {
                ReadHow::Direct
            } else {
                ReadHow::Dropped
            },
            "O_DIRECT when the filesystem takes it, the fallback when not"
        );
    }

    #[test]
    fn the_read_leg_falls_back_to_dropping_the_cache_and_reading_buffered() {
        let dir = tempfile::tempdir().unwrap();
        let (path, len) = fixture(dir.path());
        let mut buf = AlignedBuf::new(CHUNK, DIRECT_ALIGN);
        let (got, _, how) = read_back(&path, &mut buf, false).unwrap();
        assert_eq!(got, len);
        assert_eq!(how, ReadHow::Dropped);
        assert!(
            read_back(Path::new("/nonexistent/korp/fixture"), &mut buf, false).is_err(),
            "a missing file is an error, not a panic"
        );
    }

    #[test]
    fn a_full_run_names_how_it_read() {
        let dir = tempfile::tempdir().unwrap();
        let out = run_sized(dir.path(), 4 << 20, 0, Duration::from_millis(20));
        let Outcome::Done(r) = out else {
            panic!("{out:?}")
        };
        assert!(matches!(r.read_how, ReadHow::Direct | ReadHow::Dropped));
        assert!(r.seq_read_bps > 0.0);
    }

    #[test]
    fn a_missing_root_is_a_failure_row_not_a_panic() {
        let out = run_sized(
            Path::new("/nonexistent/korp"),
            4 << 20,
            0,
            Duration::from_millis(1),
        );
        assert!(matches!(out, Outcome::Failed(_)), "{out:?}");
    }
}

/// **The repeatability of this test, measured, not assumed.**
///
/// The twin election (`private-gunnar-ops`, `cargo xtask elect twin`) refuses
/// when two boxes' disks differ by less than the noise, so somebody has to
/// know what the noise IS. This is how it was measured: N production-sized
/// runs back to back on one idle filesystem, printing the spread of each leg
/// as a percentage of its own median. `--ignored` because it takes minutes
/// and writes gigabytes; it is a MEASUREMENT, re-runnable on any box.
///
/// ```text
/// cargo test -p gunnar-server --lib disktest::noise -- --ignored --nocapture
/// ```
#[cfg(test)]
mod noise {
    use super::*;

    #[test]
    #[ignore = "a measurement: minutes, and gigabytes written"]
    fn the_spread_of_ten_runs_on_one_idle_disk() {
        let runs: usize = std::env::var("GUNNAR_DISKTEST_RUNS")
            .ok()
            .and_then(|v| v.parse().ok())
            .unwrap_or(10);
        let dir = match std::env::var_os("GUNNAR_DISKTEST_FIXTURE_DIR") {
            Some(b) => tempfile::tempdir_in(PathBuf::from(b)).unwrap(),
            None => tempfile::tempdir().unwrap(),
        };
        let mut w = Vec::with_capacity(runs);
        let mut r = Vec::with_capacity(runs);
        let mut s = Vec::with_capacity(runs);
        for i in 0..runs {
            match run_sized(dir.path(), FILE_BYTES, 0, SYNC_WINDOW) {
                Outcome::Done(rep) => {
                    println!("run {i:2}  {}", rep.row());
                    w.push(rep.seq_write_bps);
                    r.push(rep.seq_read_bps);
                    s.push(rep.sync_ops_per_s);
                }
                other => panic!("run {i}: {other:?}"),
            }
        }
        for (name, mut v) in [("seq write", w), ("seq read", r), ("4k sync", s)] {
            v.sort_by(|a, b| a.partial_cmp(b).unwrap());
            let median = v[v.len() / 2];
            let (lo, hi) = (v[0], v[v.len() - 1]);
            println!(
                "{name:<10} median {median:>14.0}  min {lo:>14.0}  max {hi:>14.0}  \
                 spread {:>6.1}% of median",
                (hi - lo) / median * 100.0
            );
        }
    }
}