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> openSUSE has automatic snapshots with a fairly extensive retention policy. Ah, so Fedora is limiting it's use of snapshots to avoid the need to have balance

• cmurf 6y ago
  There is no automatic snapshotting regime on Fedora. There's no direct correlation between having many snapshots, and needing to balance. The once per month balance used in openSUSE is to preempt or reduce the chances of out of space error in one type of chunk (block group of extents), while there remains significant free space in another type of chunk. Chunks are created dynamically, and are either type metadata or data (also system but it can be ignored). Different workloads have different data/metadata ratio demands, hence dynamic allocation. Snapshots are almost entirely metadata. More snapshotting means more usage of metadata. If the pattern dramatically changes, the ratio also changes, and the dynamic allocation can alter course. Except when the disk is fully allocated. In that case, heavy metadata writes will completely fill metadata chunks, and ENOSPC even though there's still unused space in data chunks. A filtered balance can move extents from one chunk to another, and once a chunk is empty, it can be deallocated. That unallocated space can now be allocated into a different type of chunk, thus avoiding ENOSPC. Or at least when ENOSPC happens, there's essentially no free space in either chunk type, at the same time. A "true" ENOSPC. There are all kinds of mitigations for the problem in newer kernels. And in my opinion it's better to not paper over problems, but fixing the remaining edge cases.