In the old mutation logs, a version's mutations are serialized as a buffer.
Then the buffer is split into smaller chunks, e.g., 10000 bytes each. When
writting chunks to the final mutation log file, these chunks can be flushed
out of order. For instance, the (version, chunck_part) can be in the order of
(3, 0), (4, 0), (3, 1). As a result, the decoder must read forward to find all
chunks of data for a version.
Another complication is that the files are organized into blocks, where (3, 1)
can be in a subsequent block. This change checks the value size for each
version, if the size is smaller than the right size, the decoder will look
for the missing chucks in the next block.
fdbconvert is intended to convert new backup files which are tagged mutation
logs to old backup format. The actual conversion is not included in this commit
and will be added in future commits.
Note that the BackupContainer needs to be updated to support new backup files,
which is also not included in this commit.
Jingyu Zhou