On Aug 31, 2018, at 10:40 AM, Shehbaz Jaffer <shehbazjaffer007@xxxxxxxxx> wrote: > > Hi All, > > I am trying to study how backup superblocks are updated and used in > the ext4 file system. I create a 2GB file system. This creates 5 > backup superblocks with the sparse sb option. I then check the diff of > the backup blocks before and after multiple mount, write(2) fsync(2) > and close(2) and unmount operations. > > I can see the primary superblock get updated but I do not see the > backup super blocks being updated. My intuition is that the backup > blocks are only present so that the recovery can be done by replaying > the journal on the backup superblock. Are they updated each time the > journal gets full so that the "refreshed" journal can now be replayed > on updated backup superblock in case of a crash? > If this is incorrect, at what frequency do backup superblocks updated? > > If we compare this behavior with BtrFS, I can see that for each update > on fs tree, the primary block (at offset 64KB) and backup superblock > (at 64MB) gets updated. The backup ext4 superblocks are never updated by the kernel, only after a successful e2fsck, tune2fs, resize2fs, or other userspace operation. This avoids clobbering the backups with bad data if the kernel has a bug or device error (e.g. bad cable, HBA, etc). Cheers, Andreas
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