Why telescope:clear Is Slow and How to Reclaim Disk in Seconds

A while back I wrote about laravel-telescope-flusher - a tiny package I built to wipe Telescope data without waiting forever. It just hit 1,000 installs on Packagist 🎉, so it felt like the right time to actually back up the original post with real numbers, not just claims.

So I sat down, seeded a million Telescope entries on a fresh MySQL 8.0, and timed the three things you'd reach for: telescope:clear, telescope:prune, and telescope:flush. Spoiler: the gap is bigger than I expected.

Quick recap of why telescope:clear is slow

Two things kill it. First, the loop:

// vendor/laravel/telescope/src/Storage/DatabaseEntriesRepository.php
public function clear()
{
    do {
        $deleted = $this->table('telescope_entries')->take($this->chunkSize)->delete();
    } while ($deleted !== 0);
    // ...same for telescope_monitoring
}

$chunkSize = 1000. A million rows = a thousand round-trip DELETE statements, each writing to redo log, undo log, double-write buffer.

Second (and this one I missed in the original post): telescope_entries_tags has a foreign key on entry_uuid with ON DELETE CASCADE. With ~3 tags per entry, every parent delete triggers a cascade delete on the tag table. On a million entries, that's 3 million extra deletes the loop never asked for.

telescope:prune --hours=24 is the same loop with a WHERE filter. Same problem.

And DELETE doesn't give you the disk back

I missed this on the first pass. After telescope:clear finishes, information_schema.tables reports the data length as basically zero. Looks done. Then check the actual file:

ls -lah /var/lib/mysql/telescope_test/telescope_*.ibd

The .ibd files are still huge. InnoDB doesn't return space to the OS after DELETE - it only marks pages reusable for future inserts. To actually shrink the file you need OPTIMIZE TABLE (which rebuilds it) or ALTER TABLE ... ENGINE=InnoDB.

telescope:clear does neither. So your dev disk stays full.

The benchmark

Setup: MySQL 8.0 in Docker, default config. Seed: 1,000,000 telescope_entries (~2 KB JSON content each), 3,000,000 rows in telescope_entries_tags, real foreign key with cascade. Bench script lives in bench/ - go run it yourself.

Starting state, identical for both runs:

telescope_entries          rows=1000000   logical=2.33 GB   .ibd=2.4 GB
telescope_entries_tags     rows=3000000   logical=672 MB    .ibd=688 MB
telescope_monitoring       rows=50        logical=16 KB     .ibd=112 KB
TOTAL                      rows=4000050   logical=2.99 GB   .ibd=3.1 GB

Results:

Step	telescope:clear	telescope:flush
Wall time	9025 s (≈150 min)	1.21 s
Logical size after	128 KB	128 KB
.ibd files on disk after	3.1 GB (unchanged)	428 KB

That's roughly 7400× faster and 3 GB of disk you actually get back. Both runs leave info_schema reporting the same size, by the way. That's the trap. Only ls -lah on the .ibd files tells you the truth.

prune --hours=0 benches almost identically to clear (same loop, same FK cascade), so I didn't bother running it to completion. The shape of the result is the same.

What flush does differently

The package's whole command is short enough to paste:

DB::getSchemaBuilder()->withoutForeignKeyConstraints(function () {
    DB::table('telescope_entries')->truncate();
    DB::table('telescope_entries_tags')->truncate();
    DB::table('telescope_monitoring')->truncate();
});

if (DB::getDriverName() === 'mysql') {
    DB::statement('OPTIMIZE TABLE telescope_entries');
}

No magic. TRUNCATE is a metadata operation. Instant on InnoDB, no per-row work, no cascade. The withoutForeignKeyConstraints wrapper is needed because TRUNCATE doesn't fire cascades, so you have to disable the FK check yourself. OPTIMIZE TABLE rebuilds the table on innodb_file_per_table (the default for years) and produces a fresh, tiny .ibd.

There's also an App::isLocal() guard - TRUNCATE is irreversible, you really don't want to fat-finger this anywhere except dev.

When to use what

Approach	Use case
telescope:clear	Default local cleanup. Works, slow on big tables, leaves disk allocated.
telescope:prune --hours=24	Scheduled retention - keep last N hours. Same disk problem, but table size stays bounded over time.
telescope:flush (package)	Dev nuke. Telescope ballooned, you want it gone in a second and the disk back.

I don't run Telescope in production, neither should you, so the local-only guard isn't a limitation.

TL;DR

• telescope:clear = chunked DELETE LIMIT 1000 + cascading FK on tags. On 1M entries: 2.5 hours.
• InnoDB doesn't shrink the .ibd after DELETE. info_schema lies, ls -lah doesn't.
• telescope:flush = TRUNCATE + OPTIMIZE TABLE. 1.21 s on the same data, 3 GB → 428 KB on disk.
• If your info_schema says the table is empty but df disagrees, it's the InnoDB pages, not your imagination.

Resources

• 👉 Package on GitHub
• Package on Packagist
• Original post (the "why" without numbers)
• MySQL docs: OPTIMIZE TABLE

Author's Note

Thanks for sticking around!
Find me on dev.to, linkedin, or you can check out my work on github.

Notes from real-world Laravel.

Comments

[david duymelinck]

Congratulations on the 1000 installs!

I dove into the Telescope code, because I found it strange that they don't use TRUNCATE. And I found a commit where they did use it.
But they reverted it.

So I'm wondering how will your package handle the same problem that caused them to abandon the TRUNCATE code?

[Ivan Mykhavko]

Hi David, thanks for surfacing the PRs!

PR 1507 only truncated telescope_entries and telescope_monitoring with FK checks off, so TRUNCATE never cascaded and telescope_entries_tags quietly kept growing, obvious a bug.

telescope:flush just truncates all three explicitly:

DB::getSchemaBuilder()->withoutForeignKeyConstraints(function () {
    DB::table('telescope_entries')->truncate();
    DB::table('telescope_entries_tags')->truncate();
    DB::table('telescope_monitoring')->truncate();
});

No orphans. And OPTIMIZE TABLE on top reclaims the .ibd disk DELETE never gives back 😅

Cheers, and thanks for the careful read!