Hi!

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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.  

If this is a desktop, then I'm not sure what to suggest.  But for

This is a desktop.

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!

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

Hmm. I guess I could just set "read-only compatible" bit "this device
has a cache"... but last time I checked, ext3 still replayed log
during read-only mount...
									Pavel

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