diff --git a/packages/telemetry/src/flight-recorder.js b/packages/telemetry/src/flight-recorder.js
new file mode 100644
index 00000000000..b1b31a32386
--- /dev/null
+++ b/packages/telemetry/src/flight-recorder.js
@@ -0,0 +1,205 @@
+// @ts-check
+/* global BigUint64Array */
+/// <reference types="ses" />
+
+// https://github.com/Agoric/agoric-sdk/issues/3742#issuecomment-1028451575
+// I'd mmap() a 100MB file, reserve a few bytes for offsets, then use the rest
+// as a circular buffer to hold length-prefixed records. The agd process would
+// keep writing new events into the RAM window and updating the start/end
+// pointers, with some sequencing to make sure the record gets written before
+// the pointer is updated. Then, no mattter how abruptly the process is
+// terminated, as long as the host computer itself is still running, the on-disk
+// file would contain the most recent state, and anybody who reads the file will
+// get the most recent state. The host kernel (linux) is under no obligation to
+// flush it to disk any particular time, but knows when reads happen, so there's
+// no coherency problem, and the speed is unaffected by disk write speeds.
+
+import BufferFromFile from 'bufferfromfile';
+import fs from 'fs';
+import path from 'path';
+
+const { details: X } = assert;
+
+export const DEFAULT_CIRCULAR_BUFFER_SIZE = 100 * 1024 * 1024;
+export const DEFAULT_CIRCULAR_BUFFER_FILE = 'flight-recorder.bin';
+export const SLOG_MAGIC = 0x21474f4c532d4741n; // 'AG-SLOG!'
+
+const I_MAGIC = 0;
+const I_ARENA_SIZE = 1;
+const I_CIRC_START = 2;
+const I_CIRC_END = 3;
+const HEADER_LENGTH = 4;
+
+export const makeMemoryMappedCircularBuffer = ({
+  circularBufferSize = DEFAULT_CIRCULAR_BUFFER_SIZE,
+  stateDir = '/tmp',
+  circularBufferFile,
+}) => {
+  const bufferFile =
+    circularBufferFile || `${stateDir}/${DEFAULT_CIRCULAR_BUFFER_FILE}`;
+  // console.log({ circularBufferFile, bufferFile });
+
+  // If the file doesn't exist, or is not large enough, create it.
+  let stbuf;
+  try {
+    stbuf = fs.statSync(bufferFile);
+  } catch (e) {
+    if (e.code !== 'ENOENT') {
+      throw e;
+    }
+  }
+  const arenaSize = BigInt(
+    circularBufferSize - HEADER_LENGTH * BigUint64Array.BYTES_PER_ELEMENT,
+  );
+  if (!stbuf || stbuf.size < BigUint64Array.BYTES_PER_ELEMENT * 3) {
+    // Write the header.
+    const header = new Array(HEADER_LENGTH).fill(0n);
+    header[I_MAGIC] = SLOG_MAGIC;
+    header[I_ARENA_SIZE] = arenaSize;
+    fs.mkdirSync(path.dirname(bufferFile), { recursive: true });
+    fs.writeFileSync(bufferFile, BigUint64Array.from(header));
+  }
+  if (!stbuf || stbuf.size < circularBufferSize) {
+    fs.truncateSync(bufferFile, circularBufferSize);
+  }
+
+  /** @type {Uint8Array} */
+  const fileBuf = BufferFromFile(bufferFile).Uint8Array();
+  const header = new BigUint64Array(fileBuf.buffer, 0, HEADER_LENGTH);
+
+  assert.equal(
+    SLOG_MAGIC,
+    header[I_MAGIC],
+    X`${bufferFile} is not a slog buffer; wanted magic ${SLOG_MAGIC}, got ${header[I_MAGIC]}`,
+  );
+  assert.equal(
+    arenaSize,
+    header[I_ARENA_SIZE],
+    X`${bufferFile} arena size mismatch; wanted ${arenaSize}, got ${header[I_ARENA_SIZE]}`,
+  );
+  const arena = new Uint8Array(
+    fileBuf.buffer,
+    header.byteLength,
+    Number(arenaSize),
+  );
+
+  /**
+   * @param {Uint8Array} data
+   * @param {number} [offset]
+   */
+  const readCircBuf = (data, offset = 0) => {
+    assert(
+      offset + data.byteLength <= arenaSize,
+      X`Reading past end of circular buffer`,
+    );
+
+    // Read the data to the end of the arena.
+    let firstReadLength = data.byteLength;
+    const circStart = Number(header[I_CIRC_START]);
+    const readStart = (circStart + offset) % Number(arenaSize);
+    if (readStart > header[I_CIRC_END]) {
+      // The data is wrapped around the end of the arena, like BBB---AAA
+      firstReadLength = Math.min(
+        firstReadLength,
+        Number(arenaSize) - readStart,
+      );
+    }
+    data.set(arena.subarray(readStart, readStart + firstReadLength));
+    if (firstReadLength < data.byteLength) {
+      data.set(
+        arena.subarray(0, data.byteLength - firstReadLength),
+        firstReadLength,
+      );
+    }
+    return data;
+  };
+
+  /** @param {Uint8Array} data */
+  const writeCircBuf = data => {
+    if (BigUint64Array.BYTES_PER_ELEMENT + data.byteLength > arena.byteLength) {
+      // The data is too big to fit in the arena, so skip it.
+      const tooBigRecord = JSON.stringify({
+        type: 'slog-record-too-big',
+        size: data.byteLength,
+      });
+      data = new TextEncoder().encode(tooBigRecord);
+    }
+
+    const record = new Uint8Array(
+      BigUint64Array.BYTES_PER_ELEMENT + data.byteLength,
+    );
+    const lengthPrefix = new BigUint64Array(record.buffer, 0, 1);
+    lengthPrefix[0] = BigInt(data.byteLength);
+    record.set(data, BigUint64Array.BYTES_PER_ELEMENT);
+
+    // Check if we need to wrap around.
+    /** @type {bigint} */
+    let capacity;
+    if (header[I_CIRC_START] <= header[I_CIRC_END]) {
+      // ---AAAABBBB----
+      capacity =
+        header[I_ARENA_SIZE] - header[I_CIRC_END] + header[I_CIRC_START];
+    } else {
+      // BBB---AAAA
+      capacity = header[I_CIRC_START] - header[I_CIRC_END];
+    }
+
+    let overlap = BigInt(record.byteLength) - capacity;
+    while (overlap > 0n) {
+      // Advance the start pointer.
+      const startRecordLength = new BigUint64Array(1);
+      readCircBuf(new Uint8Array(startRecordLength.buffer));
+
+      const totalRecordLength =
+        BigInt(startRecordLength.byteLength) + // size of the length field
+        startRecordLength[0]; // size of the record
+
+      header[I_CIRC_START] =
+        (header[I_CIRC_START] + totalRecordLength) % header[I_ARENA_SIZE];
+      overlap -= totalRecordLength;
+    }
+
+    // Append the record.
+    let firstWriteLength = record.byteLength;
+    if (header[I_CIRC_START] < header[I_CIRC_END]) {
+      // May need to wrap, it's ---AAAABBBB---
+      firstWriteLength = Math.min(
+        firstWriteLength,
+        Number(header[I_ARENA_SIZE] - header[I_CIRC_END]),
+      );
+    }
+
+    const circEnd = Number(header[I_CIRC_END]);
+    arena.set(record.subarray(0, firstWriteLength), circEnd);
+    if (firstWriteLength < record.byteLength) {
+      // Write to the beginning of the arena.
+      arena.set(record.subarray(firstWriteLength, record.byteLength), 0);
+    }
+    header[I_CIRC_END] =
+      (header[I_CIRC_END] + BigInt(record.byteLength)) % header[I_ARENA_SIZE];
+  };
+
+  const writeJSON = obj => {
+    const text = JSON.stringify(obj, (key, value) => {
+      if (typeof value === 'bigint') {
+        return Number(value);
+      }
+      if (key === 'endoZipBase64') {
+        // Abridge the source bundle, since it's pretty huge.
+        return `[${value.length} characters...]`;
+      }
+      return value;
+    });
+    // Prepend a newline so that the file can be more easily manipulated.
+    const data = new TextEncoder().encode(`\n${text}`);
+    // console.log('have obj', obj);
+    writeCircBuf(data);
+  };
+
+  return { readCircBuf, writeCircBuf, writeJSON };
+};
+
+export const makeSlogSender = opts => {
+  const { writeJSON } = makeMemoryMappedCircularBuffer(opts);
+  return writeJSON;
+};
diff --git a/packages/telemetry/test/prepare-test-env-ava.js b/packages/telemetry/test/prepare-test-env-ava.js
index fdb00169daf..84dc9ee3599 100644
--- a/packages/telemetry/test/prepare-test-env-ava.js
+++ b/packages/telemetry/test/prepare-test-env-ava.js
@@ -1,6 +1,5 @@
-/* eslint-disable import/no-extraneous-dependencies */
-import '@endo/init/pre-bundle-source.js';
-import '@endo/lockdown/commit-debug.js';
+// @ts-check
+import '@endo/init';
 
 import { wrapTest } from '@endo/ses-ava';
 import rawTest from 'ava';
diff --git a/packages/telemetry/test/test-flight-recorder.js b/packages/telemetry/test/test-flight-recorder.js
new file mode 100644
index 00000000000..23db361fea2
--- /dev/null
+++ b/packages/telemetry/test/test-flight-recorder.js
@@ -0,0 +1,32 @@
+// @ts-check
+/* global BigUint64Array */
+import tmp from 'tmp';
+import { test } from './prepare-test-env-ava.js';
+
+import { makeMemoryMappedCircularBuffer } from '../src/flight-recorder.js';
+
+test('flight-recorder sanity', t => {
+  const { name: tmpFile } = tmp.fileSync();
+  console.log(tmpFile);
+  const { writeJSON: slogSender, readCircBuf } = makeMemoryMappedCircularBuffer(
+    {
+      circularBufferSize: 512,
+      circularBufferFile: tmpFile,
+    },
+  );
+  slogSender({ type: 'start' });
+
+  const len0 = new BigUint64Array(readCircBuf(new Uint8Array(8)).buffer);
+  const buf0 = readCircBuf(new Uint8Array(Number(len0[0])), 8);
+  const buf0Str = new TextDecoder().decode(buf0);
+  t.is(buf0Str, `\n{"type":"start"}`);
+
+  for (let i = 0; i < 500; i += 1) {
+    slogSender({ type: 'iteration', iteration: i });
+  }
+
+  const len1 = new BigUint64Array(readCircBuf(new Uint8Array(8)).buffer);
+  const buf1 = readCircBuf(new Uint8Array(Number(len1[0])), 8);
+  const buf1Str = new TextDecoder().decode(buf1);
+  t.is(buf1Str, `\n{"type":"iteration","iteration":490}`);
+});