Fine grained sync policy control (was: What I want in GNU OS)

[Chris Torek]

In article <CLINE.89Jun30171140 at suntan.ece.clarkson.edu>
cline at suntan.ece.clarkson.edu (Marshall Cline) posts one copy, each
separate, to comp.unix.questions and comp.unix.wizards, of the
following.  I have redirected followups to comp.unix.questions only.

>... I propose the following _mechanism_ (system calls) could be added
>to facilitate the _policy_. ...

>int num_dirty_blocks(int fd);
>/* "fd" is the file descriptor of an open file.
> * Returns the number of dirty blocks associated with the file descriptor.

There is now no system call that deals with `blocks' rather than
`bytes', and I believe this should be maintained.  (Actually, on 4.3BSD
and similar, `stat' fills in an st_blocks variable, but this is in
addition to the st_size field.)  In any case, I am not sure why one
would care how many bytes or blocks were not yet written: the only
interesting question seems to be `any' versus `none', with the object
of changing `any' into `none' if necessary.

>int sync_fd(int fd, int wait_until_done);
>/* Causes precisely one file to be "sync()"ed (see sync(3)).
[there is no sync(3); perhaps you mean sync(2)]
> * This gives users the fine-grained control over which files they
> * consider critical in the event of a system crash.

4BSD already has `fsync', which

	... causes all modified data and attributes of /fd/ to be
	moved to a permanent storage device.  This normally results
	in all in-core modified copies of buffers for the associated
	file to be written to a disk.

	/Fsync/ should be used by programs that require a file to be
	in a known state, for example, in building a simple transaction
	facility.

(/x/ represents italics).  The only difference here is that you are
forced to wait (fsync uses bwrite, not bawrite).

>typedef enum {DELAYED_WRITE, IMMEDIATE_WRITE, BAD_POLICY} sync_t;
>
>sync_t update_policy(int fd, sync_t mechanism);

SysV has an open() flag that causes writes to be immediate.  Presumably
this can also be set and cleared with fcntl(F_SETFL).  4BSD might
acquire this someday as well (it is not hard to implement).
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at mimsy.umd.edu	Path:	uunet!mimsy!chris