Re:Debian-installer, mdadm configuration and the Bad Blocks ...
by londons_explore on Tuesday September 15, 2020 @07:38AM
(#24480062)
To add to this, even though bad blocks are allowed by the contract between a device and the OS, because they are so rare, all code paths that deal with them are riddled with bugs.
For example, if you power off a disk improperly, all un-acked writes are allowed to contain a mix of the new data, the old data, or an unreadable sector.
If, on Linux, you now try to read one of those unreadable sectors, you will get an IO error - as designed. If you now try to write that sector, you won't be able to. Why? Well the kernel cache is 4k sectors, whereas the disk is 512 byte sectors, so to write a disk sector it needs to read the other nearby sectors to populate the cache.
That pretty much means every time you power off a disk improperly, you'll get a bunch of unreadable data and filesystem errors, even though the filesystem is journaling so should be able to cope with a sudden poweroff just fine.
For example, if you power off a disk improperly, all un-acked writes are allowed to contain a mix of the new data, the old data, or an unreadable sector.
If, on Linux, you now try to read one of those unreadable sectors, you will get an IO error - as designed. If you now try to write that sector, you won't be able to. Why? Well the kernel cache is 4k sectors, whereas the disk is 512 byte sectors, so to write a disk sector it needs to read the other nearby sectors to populate the cache.
That pretty much means every time you power off a disk improperly, you'll get a bunch of unreadable data and filesystem errors, even though the filesystem is journaling so should be able to cope with a sudden poweroff just fine.