Re: vfs: Add MS_FLUSHONFSYNC mount flag

[Eric Sandeen <sandeen@redhat.com>]

Fernando Luis Vázquez Cao wrote:
> This mount flag will be used to determine whether the block device's write
> cache should be flush or not on fsync()/fdatasync().
> 
> Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
> ---

Again, apologies for chiming in late.

But wouldn't it be better to make this a block device property rather
than a new filesystem mount option?

That way the filesystem can always do "the right thing" and call the
blkdev flush on fsync.

The block device *could* choose to ignore this in hardware if it knows
it's built with a nonvolatile write cache or if it has no write cache.

Somewhere in the middle, if an administrator knows they have a UPS they
trust and hardware that stays connected to it, they could tune the bdev
to ignore these flush requests.

Also that way if you have 8 partitions on a battery-backed blockdev, you
can tune it once, instead of needing to mount all 8 filesystems with the
new option.

Thoughts?

Thanks,
-Eric

> diff -urNp linux-2.6.29-rc3-orig/fs/namespace.c linux-2.6.29-rc3/fs/namespace.c
> --- linux-2.6.29-rc3-orig/fs/namespace.c	2009-01-29 09:47:51.000000000 +0900
> +++ linux-2.6.29-rc3/fs/namespace.c	2009-01-29 18:40:04.000000000 +0900
> @@ -1933,8 +1933,8 @@ long do_mount(char *dev_name, char *dir_
>  	if (flags & MS_RDONLY)
>  		mnt_flags |= MNT_READONLY;
>  
> -	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
> -		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT);
> +	flags &= ~(MS_NOSUID | MS_NODEV | MS_NOEXEC | MS_NOATIME |
> +		   MS_NODIRATIME | MS_RELATIME | MS_ACTIVE | MS_KERNMOUNT);
>  
>  	/* ... and get the mountpoint */
>  	retval = kern_path(dir_name, LOOKUP_FOLLOW, &path);
> diff -urNp linux-2.6.29-rc3-orig/include/linux/fs.h linux-2.6.29-rc3/include/linux/fs.h
> --- linux-2.6.29-rc3-orig/include/linux/fs.h	2009-01-29 10:04:18.000000000 +0900
> +++ linux-2.6.29-rc3/include/linux/fs.h	2009-01-29 10:24:14.000000000 +0900
> @@ -135,6 +135,8 @@ struct inodes_stat_t {
>  #define MS_RELATIME	(1<<21)	/* Update atime relative to mtime/ctime. */
>  #define MS_KERNMOUNT	(1<<22) /* this is a kern_mount call */
>  #define MS_I_VERSION	(1<<23) /* Update inode I_version field */
> +#define MS_FLUSHONFSYNC (1<<24)	/* Force block device flush on
> +				   fsync()/fdatasync() */
>  #define MS_ACTIVE	(1<<30)
>  #define MS_NOUSER	(1<<31)
>  
> 
>