One big problem that arises in data protection is error detection. One approach is to perform error detection lazily-that is, wait until a file is accessed, and at that point, check it and make sure the correct data is there. The problem with this approach is that files that are not accessed frequently may thus slowly rot away, and when finally accessed, have too many errors to be corrected. Hence, an eager approach is to perform what is sometimes called disk scrubbing- periodically go through all data and find errors proactively.
a. Assume that bit flips occur independently, at a rate of 1 flip per GB of data per month. Assuming the same 20 GB volume that is half full, and assuming that you are using the SCSI disk as specified in Figure 6.3 (4 ms seek, roughly 100 MB/sec transfer), how often should you scan through files to check and repair their integrity?
b. At what bit flip rate does it become impossible to maintain data integrity? Again assume the 20 GB volume and the SCSI disk.