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!
F137Runing 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.