tree ae2b54a1a26a89fece49f6b6d6dff8448efab542
parent eba12c7bfcb4855fc757357e5e5b0b9a474499ba
author Sage Weil <sage@newdream.net> 1213106859 -0400
committer Chris Mason <chris.mason@oracle.com> 1222355043 -0400

Btrfs: transaction ioctls

These ioctls let a user application hold a transaction open while it
performs a series of operations.  A final ioctl does a sync on the fs
(closing the current transaction).  This is the main requirement for
Ceph's OSD to be able to keep the data it's storing in a btrfs volume
consistent, and AFAICS it works just fine.  The application would do
something like

	fd = ::open("some/file", O_RDONLY);
	::ioctl(fd, BTRFS_IOC_TRANS_START);
	/* do a bunch of stuff */
	::ioctl(fd, BTRFS_IOC_TRANS_END);
or just
	::close(fd);

And to ensure it commits to disk,

	::ioctl(fd, BTRFS_IOC_SYNC);

When a transaction is held open, the trans_handle is attached to the
struct file (via private_data) so that it will get cleaned up if the
process dies unexpectedly.  A held transaction is also ended on fsync() to
avoid a deadlock.

A misbehaving application could also deliberately hold a transaction open,
effectively locking up the FS, so it may make sense to restrict something
like this to root or something.

Signed-off-by: Chris Mason <chris.mason@oracle.com>