LP #1570682: Parallel doublewrite buffer file created when skip-innodb_doublewrite is set

**Reported in Launchpad by Sean last update 04-05-2016 10:05:05

I use Percona on top of ZFS, so I disable the doublewrite buffer since it's redundant and unnecessary. I do this with skip-innodb_doublewrite in /etc/mysql/my.cnf.

The new Percona 5.7.11-4 has added the new parallel doublewrite buffer feature and according to the log file (next line), this new buffer is being created.

[Note] InnoDB: Created parallel doublewrite buffer at /var/lib/mysql/xb_doublewrite, size 31457280 bytes

I don't think it's actually being used, since an "ls -l" in /var/lib/mysql shows a stale "last updated" timestamp for the xb_doublewrite file.

But it should not be created at all if doublewrites are disabled.