On Tue, 25 Jul 2017, Pavel Machek wrote: > On Tue 2017-07-25 12:32:48, Vojtech Pavlik wrote: > > 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. > > 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. Note that if bcache comes up without its cache, you will need to: echo 1 > /sys/block/sdX/bcache/running This is of course unsafe with writeback but should be fine with writethrough. -- Eric Wheeler > > > 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. > > My plan is to make offset 0. AFAICT bcache superblock can be shrunk. > > Pavel > -- > (english) http://www.livejournal.com/~pavelmachek > (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html >
