Tried to migrate from mysql blob storage to local disk and got a little over 1200 failures. On investigating this was the trace for one example:
```
./bin/files migrate --engine local-disk F137 --trace
>>> [2] <connect> phabricator_file
<<< [2] <connect> 2,626 us
>>> [3] <query> SELECT f.* FROM `file` f WHERE (f.id IN (137)) ORDER BY f.id DESC
<<< [3] <query> 577 us
>>> [4] <query> SELECT edge.* FROM `edge` edge WHERE (edge.src IN ('PHID-FILE-vj7icvfermee5m7qie5a')) AND (edge.type IN ('26')) ORDER BY edge.dateCreated DESC, edge.seq DESC
<<< [4] <query> 579 us
F137: Migrating from 'blob' to 'local-disk'...>>> [5] <query> SELECT * FROM `file_storageblob` WHERE `id` = 7
<<< [5] <query> 656 us
failed!
exception 'Exception' with message 'Unable to load MySQL blob file '7'!' in /home/phabricator/phabricator/src/applications/files/engine/PhabricatorMySQLFileStorageEngine.php:75
Stack trace:
#0 /home/phabricator/phabricator/src/applications/files/engine/PhabricatorMySQLFileStorageEngine.php(49): PhabricatorMySQLFileStorageEngine->loadFromMySQLFileStorage('7')
#1 /home/phabricator/phabricator/src/applications/files/storage/PhabricatorFile.php(479): PhabricatorMySQLFileStorageEngine->readFile('7')
#2 /home/phabricator/phabricator/src/applications/files/storage/PhabricatorFile.php(317): PhabricatorFile->loadFileData()
#3 /home/phabricator/phabricator/src/applications/files/management/PhabricatorFilesManagementMigrateWorkflow.php(82): PhabricatorFile->migrateToEngine(Object(PhabricatorLocalDiskFileStorageEngine))
#4 /home/phabricator/libphutil/src/parser/argument/PhutilArgumentParser.php(394): PhabricatorFilesManagementMigrateWorkflow->execute(Object(PhutilArgumentParser))
#5 /home/phabricator/libphutil/src/parser/argument/PhutilArgumentParser.php(290): PhutilArgumentParser->parseWorkflowsFull(Array)
#6 /home/phabricator/phabricator/scripts/files/manage_files.php(21): PhutilArgumentParser->parseWorkflows(Array)
#7 {main}
Failures!
F137
```
Runing the queries shown individually shows that the id exists in the `file` table, but an empty set is returned from the `edge` table.
I'm not sure how the relationship in question was lost, or perhaps never existed, but am curious if it's possible to migrate these remaining files.