On Mon, Aug 17, 2020 at 03:54:29AM +0000, Rogério Brito wrote: > Dear developers, > > I have been using ext4 for quite some time and I have some ext4 > filesystems that led me to some questions. > > I have at least 2 "large" filesystems (with 2TB each, both almost > full) dating back from 2011 and 2010. I believe that I even converted > one of them from ext3 to ext4, but my memory is not that clear after > almost a decade. As such, they were created without some useful > features that are useful nowadays, like inline files. With that in > mind, here are some questions: > > 1 - I know that some features can be enabled with tune2fs, but, in > particular, inline files don't seem to be. I've seen some people > indicate that using debugfs, I can mark the superblock as having > support for it. Would that really work? I don't plan on booting old > kernels. One of those filesystems is running on an armel device that > is quite slow and I would really like to avoid copying all the files > to an external HD, recreating the filesystem and, then, copying back > the files to the system. Inline data is not enabled by default, because there are still a few corner cases that will cause stress tests to fail. So it's not something I'd encourage enabling; it doesn't make that much difference unless your workload creates huge numbers of small (< ~100 bytes) files. If you do create lots of small files, but then don't attempt to append to them, or truncate them, etc., later., inline_data will work, but it's likely that the presence or absence of inline_data probably won't make that much difference to your application, unless you're doing something super unusual. > 2 - Is there any way to get transparent compression with ext4? That > would really, really rock and is, perhaps, one of the features that > some users like me would greatly benefit from. No, transparent compression is not a feature ext4 has. There has been some thinking about creating a read-only compression feature, primarily to speed up reading from slow devices, but full transparent compression where users might want to seek to the middle of a transparently compressioned file, and then writing into the middle of said file, and making sure this is reliable even in the face of a crash in the middle of doing such an update, etc., is something that would be a large amount of work, for relatively little benefit, and so I'd be surprised if that were ever to be supported in ext4. Read-only compressed files is something that might happen at some point, but while we have a design sketched out for it, no one is currently actively working on that feature. Cheers, - Ted
