Re: bcache with existing ext4 filesystem

[Austin S. Hemmelgarn]

On 2017-07-26 13:41, Eric Wheeler wrote:
> On Wed, 26 Jul 2017, Pavel Machek wrote:
>
> > Hi!
> >
> > > > > > Unfortunately, that would mean shifting 400GB data 8KB forward, and
> > > > > > compatibility problems. So I'd prefer adding bcache superblock into
> > > > > > the reserved space, so I can have caching _and_ compatibility with
> > > > > > grub2 etc (and avoid 400GB move):
> > > > >
> > > > > The common way to do that is to move the beginning of the partition,
> > > > > assuming your ext4 lives in a partition.
> > > >
> > > > Well... if I move the partition, grub2 (etc) will be unable to access
> > > > data on it. (Plus I do not have free space before some of the
> > > > partitions I'd like to be cached).
> > >
> > > Why not use dm-linear and prepend space for the bcache superblock?  If
> > > this is your boot device, then you would need to write a custom
> > > initrd hook too.
> >
> > Thanks for a pointer. That would actually work, but I'd have to be
> > very, very careful using it...
> >
> > ...because if I, or systemd or some kind of automounter sees the
> > underlying device (sda4) and writes to it (it is valid ext4 after
> > all), I'll have inconsistent base device and cache ... and that will
> > be asking for major problems (even in writethrough mode).
>
> Sigh.  Gone are the days when distributions would only mount filesystems
> if you ask them to.
This is only an issue if:
1. You are crazy and have your desktop set up to auto-mount stuff on 
insertion (instead of on-access), or you're using a brain-dead desktop 
that doesn't let you turn this functionality off.
or:
2. You are mounting by filesystem label or filesystem UUID 
(/dev/mapper/* and /dev/<vgname>/<lvname> links are more than 
sufficiently stable unless you're changing the storage stack all the 
time).  This is of course almost always the case these days because for 
some reason the big distros assume that the device mapper links aren't 
stable or aren't safe to use.
or:
3. You're working with a multi-device BTRFS volume (and in that case, 
it's not an issue of auto-mounting it, but an issue of buggy kernel 
behavior combined with the 'scan everything for BTRFS' udev rule that's 
now upstream in udev).
> If this is a desktop, then I'm not sure what to suggest.  But for server
> with no GUI, turn off the grub2 osprober (GRUB_DISABLE_OS_PROBER="true" in
> /etc/sysconfig/grub).  If this was LVM, then I would suggest also setting
> global_filter in lvm.conf.  If you find other places that need poked to
> prevent automounting then please let me know!
Nope, just those and what I mentioned above under 1 and 2.
> As for ext4 feature bits, can they be arbitrarily named? (I think they are
> bits, so maybe not).  Maybe propose a patch to ext4 to provide a
> "disable_mount" feature.  This would prevent mounting altogether, and you
> would set/clear it when you care to.  A strange feature indeed.  Doing
> this as an obscure feature on a single filesystem doesn't quite seem
> right.
In the case of ext4, you can use the MMP feature (with the associated 
minor overhead) to enforce this.
> It might be better to have a block-layer device-mount mask so devices that
> are allowed to be mounted can be whitelisted on the kernel cmdline or
> something.  blk.allow_mount=8:16,253:*,...  or blk.disallow_mount=8:32 (or
> probably both).
>
> Just ideas, but it would be great to allow mounting only those
> major/minors which are authorized, particularly with increasingly complex
> block-device stacks.
If it's not explicitly created as a mount unit, fstab entry, or 
automount entry, and you're not using a brain-dead desktop, and you're 
not using FS labels or UUID's to mount, it _really_ isn't an issue 
except on BTRFS, and for that this would provide _no_ benefit, because 
the issue there results from confusion over which device to use combined 
with the assumption that a UUID stored on the device is sufficient to 
uniquely identify a filesystem.