A New Problem: Misdirected Writes

The basic scheme described in the previous lesson works well in the general case of corrupted blocks. However, modern disks have a couple of unusual failure modes that require different solutions.

The first failure mode of interest is called a misdirected write. This arises in disk and RAID controllers which write the data to disk correctly, except in the wrong location. In a single-disk system, this means that the disk wrote block $D_x$ not to address $x$ (as desired) but rather to address $y$ (thus “corrupting” $D_y$). In addition, within a multi-disk system, the controller may also write $D_{i,x}$ not to address $x$ of disk $i$ but rather to some other disk $j$. Thus our question:

CRUX: HOW TO HANDLE MISDIRECTED WRITES

How should a storage system or disk controller detect misdirected writes? What additional features are required from the checksum?

Adding a physical identifier

The answer, not surprisingly, is simple: add a little more information to each checksum. In this case, adding a physical identifier (physical ID) is quite helpful. For example, if the stored information now contains the checksum $C(D)$ and both the disk and sector numbers of the block, it is easy for the client to determine whether the correct information resides within a particular locale. Specifically, if the client is reading block 4 on disk 10 ($D_{10,4}$), the stored information should include that disk number and sector offset, as shown below. If the information does not match, a misdirected write has taken place, and a corruption is now detected. Here is an example of what this added information would look like on a two-disk system. Note that this figure, like the others before it, is not to scale, as the checksums are usually small (e.g., 8 bytes) whereas the blocks are much larger (e.g., 4 KB or bigger):