In commit 24648768b4 (MDEV-30136)
the parameter innodb_flush_method was deprecated, with no direct
replacement for innodb_flush_method=O_DIRECT_NO_FSYNC.
Let us change innodb_doublewrite from Boolean to ENUM that can
be changed while the server is running:
OFF: Assume that writes of innodb_page_size are atomic
ON: Prevent torn writes (the default)
fast: Like ON, but avoid synchronizing writes to data files
The deprecated start-up parameter innodb_flush_method=NO_FSYNC will cause
innodb_doublewrite=ON to be changed to innodb_doublewrite=fast,
which will prevent InnoDB from making any durable writes to data files.
This would normally be done right before the log checkpoint LSN is updated.
Depending on the file systems being used and their configuration,
this may or may not be safe.
The value innodb_doublewrite=fast differs from the previous combination of
innodb_doublewrite=ON and innodb_flush_method=O_DIRECT_NO_FSYNC by always
invoking os_file_flush() on the doublewrite buffer itself
in buf_dblwr_t::flush_buffered_writes_completed(). This should be safer
when there are multiple doublewrite batches between checkpoints.
Typically, once per second, buf_flush_page_cleaner() would write out
up to innodb_io_capacity pages and advance the log checkpoint.
Also typically, innodb_io_capacity>128, which is the size of the
doublewrite buffer in pages. Should os_file_flush_func() not be invoked
between doublewrite batches, writes could be reordered in an unsafe way.
The setting innodb_doublewrite=fast could be safe when the doublewrite
buffer (the first file of the system tablespace) and the data files
reside in the same file system.
This was tested by running "./mtr --rr innodb.alter_kill". On the first
server startup, with innodb_doublewrite=fast, os_file_flush_func()
would only be invoked on the ibdata1 file and possibly ib_logfile0.
On subsequent startups with innodb_doublewrite=OFF, os_file_flush_func()
will be invoked on the individual data files during log_checkpoint().
Note: The setting debug_no_sync (in the code, my_disable_sync) would
disable all durable writes to InnoDB files, which would be much less safe.
IORequest::Type: Introduce special values WRITE_DBL and PUNCH_DBL
for asynchronous writes that are submitted via the doublewrite buffer.
In this way, fil_space_t::use_doublewrite() or buf_dblwr.in_use()
will only be consulted during buf_page_t::flush() and the doublewrite
buffer can be enabled or disabled without any fear of inconsistency.
buf_dblwr_t::block_size: Replaces block_size().
buf_dblwr_t::flush_buffered_writes(): If !in_use() and the doublewrite
buffer is empty, just invoke fil_flush_file_spaces() and return. The
doublewrite buffer could have been disabled while a batch was in
progress.
innodb_init_params(): If innodb_flush_method=O_DIRECT_NO_FSYNC,
set innodb_doublewrite=fast or innodb_doublewrite=fearless.
Thanks to Mark Callaghan for reporting this, and Vladislav Vaintroub
for feedback.
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