Re: [TUHS] Disk data layout (was: /dev/drum)

@Fortune Systems we kept a forwarding map of bad blocks. This was stored in a file at inode#1 IIRC. To read/write block n you check if n was in the list. If not, you read/write disk block n. Else you use the mapped block. The last N blocks were reserved for this. Actually we had to keep some state. A block that gave an error on read was marked bad so that further reads errored out. But a write to such a block would allocate a new block and add an entry in the bad block forwarding map. Further read/writes would then Go to this block. I also kept soft (correctable) error stats. These blocks were more likely to go bad so periodically a background task would preemptively forward these blocks. To test this logic I used to build the v7 kernel on a very unreliable disk! But this slowed things down a lot - having to seek to the end zone and then back for the next block. I think in later disk drives an extra block per track was reserved for this and the drive took care of this. It would’ve made more sense to avoid this forwarding by exposing bad blocks to the file system layer so that it can avoid allocating them but this would’ve complicated the interface.