On 2019-11-07 9:03 a.m., Nate Eldredge wrote: > 1. What causes this? I saw some references to "unused extents" but it > wasn't clear how that happens, or why they wouldn't be freed through > normal operation. Are there certain usage patterns that exacerbate it? Virtual Box Image files are subject to many, many small writes... (just booting windows, for example, can create well over 5000 file fragments.) When the image file is new, the extents will be very large. In BTRFS, the extents are immutable. When a small write creates a new 4K COW extent, the old 4k remains as part of the old extent as well. This situation will remain until all the data in the old extent is re-written.. when none of that data is referenced anymore, the extent will be freed. > 5. Is there a better way to detect this kind of wastage, to distinguish > it from more mundane causes (deleted files still open, etc) and see how > much space could be recovered? In particular, is there a way to tell > which files are most affected, so that I can just defragment those? Generally speaking, files that are subject to many random writes are few, and you should be well aware of the larger ones where this might be an issues,, (virtual image files, large databases, etc.) These files should be defragmented frequently. I don't see any reason not run defrag over the whole subvolume, but if you want to search for files with absurd fragments, you can always use the find command to search for files, run the filefrag command on them, then use whatever tools you like to search the output for files with thousands of fragments.
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