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. > > 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 -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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