This is a follow-up PR to #29792 to get rid of the data file sync.
**This is a non-backward compatible change, which increments the
database version from 8 to 9**.
We introduce a flushOffset for each freezer table, which tracks the position
of the most recently fsync’d item in the index file. When this offset moves
forward, it indicates that all index entries below it, along with their corresponding
data items, have been properly persisted to disk. The offset can also be moved
backward when truncating from either the head or tail of the file.
Previously, the data file required an explicit fsync after every mutation, which
was highly inefficient. With the introduction of the flush offset, the synchronization
strategy becomes more flexible, allowing the freezer to sync every 30 seconds
instead.
The data items above the flush offset are regarded volatile and callers must ensure
they are recoverable after the unclean shutdown, or explicitly sync the freezer
before any proceeding operations.
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Co-authored-by: Felix Lange <fjl@twurst.com>
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