On Tue, Jul 25, 2017 at 08:43:04AM +0200, Pavel Machek wrote: > On Tue 2017-07-25 00:51:56, Theodore Ts'o wrote: > > On Mon, Jul 24, 2017 at 10:04:51PM +0200, Pavel Machek wrote: > > > Question for you was... Is the first 1KiB of each ext4 filesystem still > > > free and "reserved for a bootloader"? > > > > Yes. > > Thanks. > > > > If I needed more for bcache superblock (8KiB, IIRC), would that be > > > easy to accomplish on existing filesystem? > > > > Huh? Why would the bcache superblock matter when you're talking about > > the ext4 layout? The bcache superblock will be on the bcache > > device/partition, and the ext4 superblock will be on the ext4 > > device/partition. > > I'd like to enable bcache on already existing ext4 partition. AFAICT > normal situation, even on the backing device, is: > > | 8KiB bcache superblock | 1KiB reserved | ext4 superblock | 400GB data | > > 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. I don't see how overlapping the ext4 and the bcache backing device starts would give you what you want, because bcache assumes the backing device data starts with an offset. -- Vojtech Pavlik
