[dm-devel] generic wrappers for multi-device FS operations

[Andreas Dilger]

On 2011-03-08, at 10:04 AM, Ric Wheeler wrote:
> After seeing some of the feedback and confusion that happened in the fedora community after Josef suggestion that we default to btrfs in an upcoming Fedora release, it became clear to me that many users are incredibly unaware of the common features that we have across file systems today given LVM/device mapper support.
> 
> btrfs will make multi-volume/multi-disk operations common place and easy to do, but there is no reason not to do most/all of this today with ext4, xfs, etc on top of lvm.
> 
> To make this trivial to do for users, I think that it would be really nice to have a two-level wrappers for things like resize, add a volume, shrink, etc. Similar to the way we have mount or fsck invoke file system specific bits.

I definitely think this makes sense.  However, taking a quick look at fsadm,
I don't think it is the right starting point for this work.  It is essentially
a single script that is special-casing each filesystem it is touching, which
makes it a maintenance nightmare to add in support for different filesystems.

A better structure is the mkfs.* and fsck.* tools that extend the basic
mkfs/fsck functionality for each new filesystem.  That allows new filesystems
to be added without the requirement to modify the upstream fsadm script.


Another tool similar to this that I've been trying to push upstream for some
time is the "lvcheck" script, which is essentially a wrapper for online
filesystem checking.  It is currently structured as an extension to the LVM
tools, since it depends on creating a snapshot of an LV and does a check on
the snapshot.  If the snapshot is clean the original filesystem is marked
checked as well, which avoids the "slow ext* check on boot" problem, while
still ensuring that periodic filesystem checks will catch latent errors.

It wouldn't be unreasonable to have a new wrapper for online filesystem
checking (e.g. ofsck) or just an extension to fsck that does this in a more
"plug-in" manner like fsck.* does today.  It would naturally progress into
real online checking for filesystems that support this (e.g. btrfs, and I
think XFS is going in this direction as well).

Cheers, Andreas