My question is, what does it do then when a new modification-write comes in to the compressed no-cow file, and the modification isn't as compressible as the data it replaced?
If any extent that is attempted to compress is found not compressible then it would mark inode as no-compress. Then inodes with no-compress flag will never attempt to compress its future COW extents. However if a use case expects future extents may be compressible then force-compress option will help, where each COW extent will be attempted to compress and holds compressed data only if there is a compression benefit.
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