diff --git a/lib/private/Snowflake/FileSequence.php b/lib/private/Snowflake/FileSequence.php
index 463dfd762e195..067bf39134993 100644
--- a/lib/private/Snowflake/FileSequence.php
+++ b/lib/private/Snowflake/FileSequence.php
@@ -13,15 +13,30 @@
 use OCP\ITempManager;
 use Override;
 
+/**
+ * Sequence IDs shared between processes through lock files in the temporary directory.
+ *
+ * Each lock file is a table of counters. To get a sequence ID, the file chosen by the
+ * millisecond (ms % NB_FILES) is locked and the slot of that second and millisecond is
+ * read: ((seconds % SEQUENCE_TTL) * (1000 / NB_FILES) + ms / NB_FILES). A slot holds the
+ * seconds it was last used for and the last sequence ID handed out. If the seconds match,
+ * the next sequence ID is that one plus one, otherwise it starts at 0. The result is
+ * written back and the file unlocked.
+ *
+ * A slot is reused SEQUENCE_TTL seconds later, the seconds mismatch then resets its counter.
+ * No fsync is needed, as the files only coordinate processes running at the same time.
+ */
 class FileSequence implements ISequence {
 	/** Number of files to use */
 	private const int NB_FILES = 20;
 	/** Lock file directory **/
 	public const LOCK_FILE_DIRECTORY = 'sfi_file_sequence';
 	/** Lock filename format **/
-	private const string LOCK_FILE_FORMAT = 'seq-%03d.lock';
-	/** Delete sequences after SEQUENCE_TTL seconds **/
+	private const string LOCK_FILE_FORMAT = 'seq-%03d.slots';
+	/** Reuse the slot of a sequence after SEQUENCE_TTL seconds **/
 	private const int SEQUENCE_TTL = 30;
+	/** Size of a slot: the seconds and the last sequence ID, both as unsigned 32-bit integers **/
+	private const int SLOT_SIZE = 8;
 
 	private string $workDir;
 
@@ -69,42 +84,26 @@ public function nextId(int $serverId, int $seconds, int $milliseconds): int {
 			throw new \Exception('Unable to acquire lock on sequence ID file: ' . $filePath);
 		}
 
-		// Read content
-		$content = (string)fgets($fp);
-		$locks = $content === ''
-			? []
-			: json_decode($content, true, 3, JSON_THROW_ON_ERROR);
-
-		// Generate new ID
-		if (isset($locks[$seconds])) {
-			if (isset($locks[$seconds][$milliseconds])) {
-				++$locks[$seconds][$milliseconds];
-			} else {
-				$locks[$seconds][$milliseconds] = 0;
+		// Each file holds one slot per second of the TTL window and per millisecond mapped to it
+		$slot = ($seconds % self::SEQUENCE_TTL) * intdiv(1000, self::NB_FILES) + intdiv($milliseconds, self::NB_FILES);
+		fseek($fp, $slot * self::SLOT_SIZE);
+		$data = fread($fp, self::SLOT_SIZE);
+		$sequenceId = 0;
+		if (is_string($data) && strlen($data) === self::SLOT_SIZE) {
+			['seconds' => $slotSeconds, 'sequence' => $slotSequenceId] = unpack('Nseconds/Nsequence', $data);
+			if ($slotSeconds === $seconds) {
+				$sequenceId = $slotSequenceId + 1;
 			}
-		} else {
-			$locks[$seconds] = [
-				$milliseconds => 0
-			];
 		}
 
-		// Clean old sequence IDs
-		$cleanBefore = $seconds - self::SEQUENCE_TTL;
-		$locks = array_filter($locks, static function ($key) use ($cleanBefore) {
-			return $key >= $cleanBefore;
-		}, ARRAY_FILTER_USE_KEY);
-
-		// Write data
-		ftruncate($fp, 0);
-		$content = json_encode($locks, JSON_THROW_ON_ERROR);
-		rewind($fp);
-		fwrite($fp, $content);
-		fsync($fp);
+		// No fsync needed, the file only coordinates processes running at the same time
+		fseek($fp, $slot * self::SLOT_SIZE);
+		fwrite($fp, pack('NN', $seconds, $sequenceId));
 
 		// Release lock
 		fclose($fp);
 
-		return $locks[$seconds][$milliseconds];
+		return $sequenceId;
 	}
 
 	private function getFilePath(int $fileId): string {
diff --git a/lib/private/Snowflake/ISequence.php b/lib/private/Snowflake/ISequence.php
index e5e1f6e414d32..22c5f52a840dc 100644
--- a/lib/private/Snowflake/ISequence.php
+++ b/lib/private/Snowflake/ISequence.php
@@ -20,6 +20,8 @@ public function isAvailable(): bool;
 
 	/**
 	 * Returns next sequence ID for current time and server
+	 *
+	 * @param non-negative-int $seconds seconds since the Snowflake epoch
 	 */
 	public function nextId(int $serverId, int $seconds, int $milliseconds): int|false;
 }
diff --git a/lib/private/Snowflake/SnowflakeGenerator.php b/lib/private/Snowflake/SnowflakeGenerator.php
index a73253d246c7f..a720a19c79595 100644
--- a/lib/private/Snowflake/SnowflakeGenerator.php
+++ b/lib/private/Snowflake/SnowflakeGenerator.php
@@ -37,11 +37,14 @@ public function nextId(?DateTimeImmutable $timestamp = null): string {
 
 		// Relative time
 		$seconds = $timestamp->getTimestamp() - self::TS_OFFSET;
+		if ($seconds < 0) {
+			throw new \InvalidArgumentException('Snowflake IDs cannot be generated for a time before ' . date(DATE_ATOM, self::TS_OFFSET));
+		}
 		$milliseconds = (int)$timestamp->format('v');
 
 		$serverId = $this->serverInfo->getServerId();
 		$isCli = (int)$this->isCli(); // 1 bit
-		$sequenceId = $this->sequenceGenerator->nextId($seconds, $milliseconds, $serverId); //  12 bits
+		$sequenceId = $this->sequenceGenerator->nextId($serverId, $seconds, $milliseconds); //  12 bits
 		if ($sequenceId > 0xFFF || $sequenceId === false) {
 			// Throttle a bit, wait for next millisecond
 			usleep(1000);
diff --git a/lib/public/Snowflake/ISnowflakeGenerator.php b/lib/public/Snowflake/ISnowflakeGenerator.php
index 475d376788313..d64bb02a1cb13 100644
--- a/lib/public/Snowflake/ISnowflakeGenerator.php
+++ b/lib/public/Snowflake/ISnowflakeGenerator.php
@@ -42,6 +42,7 @@ interface ISnowflakeGenerator {
 	 *
 	 * @param ?DateTimeImmutable $timestamp Generate the Snowflake ID for a specific time. This should only be used in very special cases.
 	 * @return non-empty-string
+	 * @throws \InvalidArgumentException if the timestamp is before 2025-10-01, as such a time cannot be represented
 	 *
 	 * @since 33.0
 	 */
diff --git a/tests/lib/Snowflake/FileSequenceTest.php b/tests/lib/Snowflake/FileSequenceTest.php
index bec2dab7b7ce5..f58c8a8213bca 100644
--- a/tests/lib/Snowflake/FileSequenceTest.php
+++ b/tests/lib/Snowflake/FileSequenceTest.php
@@ -23,18 +23,43 @@ public function setUp():void {
 		parent::setUp();
 
 		$tempManager = $this->createMock(ITempManager::class);
-		$this->path = sys_get_temp_dir();
+		$this->path = sys_get_temp_dir() . '/' . uniqid('file_sequence_test_');
+		mkdir($this->path);
 		$tempManager->method('getTempBaseDir')->willReturn($this->path);
 		$this->sequence = new FileSequence($tempManager);
 	}
 
 	#[\Override]
 	public function tearDown():void {
-		$lockDirectory = $this->path . '/' . FileSequence::LOCK_FILE_DIRECTORY;
-		foreach (glob($lockDirectory . '/*') as $file) {
+		foreach (glob($this->path . '/' . FileSequence::LOCK_FILE_DIRECTORY . '*/*') as $file) {
 			unlink($file);
 		}
+		foreach (glob($this->path . '/' . FileSequence::LOCK_FILE_DIRECTORY . '*') as $directory) {
+			rmdir($directory);
+		}
+		rmdir($this->path);
 
 		parent::tearDown();
 	}
+
+	public function testSequenceIncrementsWithinTheSameMillisecond(): void {
+		$this->assertSame(0, $this->sequence->nextId(42, 1000, 500));
+		$this->assertSame(1, $this->sequence->nextId(42, 1000, 500));
+		$this->assertSame(2, $this->sequence->nextId(42, 1000, 500));
+	}
+
+	public function testSequenceStartsAtZeroForEachMillisecond(): void {
+		$this->assertSame(0, $this->sequence->nextId(42, 1000, 500));
+		// Same lock file, different slot
+		$this->assertSame(0, $this->sequence->nextId(42, 1000, 520));
+		$this->assertSame(0, $this->sequence->nextId(42, 1001, 500));
+		// Same slot, reused after the TTL window
+		$this->assertSame(0, $this->sequence->nextId(42, 1030, 500));
+	}
+
+	public function testSequenceKeepsEarlierMillisecondsWithinTheWindow(): void {
+		$this->assertSame(0, $this->sequence->nextId(42, 1000, 500));
+		$this->assertSame(0, $this->sequence->nextId(42, 1001, 500));
+		$this->assertSame(1, $this->sequence->nextId(42, 1000, 500));
+	}
 }
diff --git a/tests/lib/Snowflake/GeneratorTest.php b/tests/lib/Snowflake/GeneratorTest.php
index a3d60f7b93efe..733612d53b52b 100644
--- a/tests/lib/Snowflake/GeneratorTest.php
+++ b/tests/lib/Snowflake/GeneratorTest.php
@@ -66,6 +66,13 @@ public function testGenerator(): void {
 		$this->assertEquals($this->serverInfo->getServerId(), $data->getServerId());
 	}
 
+	public function testGeneratorRejectsTimestampBeforeEpoch(): void {
+		$generator = new SnowflakeGenerator(new TimeFactory(), $this->sequence, $this->serverInfo);
+
+		$this->expectException(\InvalidArgumentException::class);
+		$generator->nextId(new \DateTimeImmutable('2025-09-30 23:59:59 UTC'));
+	}
+
 	public function testMinForTime(): void {
 		$generator = new SnowflakeGenerator(new TimeFactory(), $this->sequence, $this->serverInfo);
 		$now = time();