[Ceph-announce] Deprecating ext4 support

Hi, ext4 has never been recommended, but we did test it. After Jewel is out, we would like explicitly recommend *against* ext4 and stop testing it. Why: Recently we discovered an issue with the long object name handling that is not fixable without rewriting a significant chunk of FileStores filename handling. (There is a limit in the amount of xattr data ext4 can store in the inode, which causes problems in LFNIndex.) We *could* invest a ton of time rewriting this to fix, but it only affects ext4, which we never recommended, and we plan to deprecate FileStore once BlueStore is stable anyway, so it seems like a waste of time that would be better spent elsewhere. Also, by dropping ext4 test coverage in ceph-qa-suite, we can significantly improve time/coverage for FileStore on XFS and on BlueStore. The long file name handling is problematic anytime someone is storing rados objects with long names. The primary user that does this is RGW, which means any RGW cluster using ext4 should recreate their OSDs to use XFS. Other librados users could be affected too, though, like users with very long rbd image names (e.g., > 100 characters), or custom librados users. How: To make this change as visible as possible, the plan is to make ceph-osd refuse to start if the backend is unable to support the configured max object name (osd_max_object_name_len). The OSD will complain that ext4 cannot store such an object and refuse to start. A user who is only using RBD might decide they don't need long file names to work and can adjust the osd_max_object_name_len setting to something small (say, 64) and run successfully. They would be taking a risk, though, because we would like to stop testing on ext4. Is this reasonable? If there significant ext4 users that are unwilling to recreate their OSDs, now would be the time to speak up. Thanks! sage