My previous reply was incorrect in one point - the data is never copied from the transaction log into the sparse datafiles, instead the application writes the same data independently to both locations. Also, I failed to mention that the files are memmapped, and it's possible that the write operations attempt to use DIRECT_IO, which I believe is disabled by btrfs with compression - is it possible that an attempt to use DIRECT_IO or memmapped files would prevent compression?
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