Differential stores the source text for code reviews in a "hunk" table. This table has two formats, a "legacy" format and a "modern" format. The modern format is better, and supports compression and other features.
New hunks have been written to the "modern" table for a very long time (perhaps a year or more?) but installs which have been around for longer than that may still have some old hunks stored in the "legacy" format. In a few weeks, we will add a mandatory hunk format migration which migrates any legacy hunks to modern hunk storage.
For installs with a long history, this may take some time. This migration is safe to run online (while Phabricator is running), and you can potentially avoid maintenance downtime by running this migration manually in advance of the forced migration.
**Determining If You Are Affected**
To determine if you are affected, run `bin/storage probe` and look at the size of the `differential_hunk` table in the `phabricator_differential` database.
- If this table has size `0.0 MB`, your install has no legacy hunks. You are not affected at all and can ignore everything happening here.
- If this table has a small size (up to about `100 MB`), your install has some legacy hunks but the migration should happen fairly quickly (a few minutes). You can run the migration manually if you prefer, or wait for the forced migration.
- If this table is large (bigger than `100 MB`) you may be able to reduce maintenance downtime significantly by running the migration manually in advance of the forced migration.
**Manual Migration**
To run the migration manually:
- Watch this space for instructions!
- (The migration script isn't in a runnable state in the `stable` branch yet and the `stable` branch isn't documented; come back next week for a more robust path forward.)
**Context and Discussion**
See T8475 and linked objects for technical context and discussion.