The capacity and performance of hard disk systems have improved radically since those early systems that we have been talking about above, and one means by which a considerable improvement has been made is to utilize better the so-called areal densityof the disk: that is, the number of bits that can be packed into each unit of area on the disk. As we noted above, if we keep the same number of sectors per track across the whole disk then the packing density towards the centre is much greater than the packing density at the outermost edge. If we are to make the most of the allowable areal density across the whole surface of the disk, it is essential that we have different numbers of sectors per track for different groups of cylinders on the disk. Then, as we move outwards from the centre of the disk, and the track length increases, so outer cylinders could have more sectors per track than inner cylinders for the same bit density. When using this technique, of course, write precompens- ation becomes less of an issue.
Having more than one set of sectors per track on the disk is known aszoned bit recording(ZBR)23. With this approach, tracks are grouped into zones and each zone on the disk has a different number of sectors per track. As we move from the innermost zone outwards so the sectors per track figure increases such that the bit packing density within all the tracks is now more even.
At Fig. 5.10 we see the 15 zones of a 3.8 Gbyte Quantum Fireball™ hard disk (Quantum, 1996), with sectors per track ranging from 122 to 232, and with each zone
22 Clearly, if we are to get the same number of bits (because there are the same number of sectors per track) into a smaller track length, then they have to be closer together. 23 Also known asmultiple zone recording(MZR)andzoned constant angular velocity
consisting of 454 tracks24. We may wish to note the very large number of tracks25(15 × 454 = 6810) on this disk and the much larger numbers for sectors per track26 compared with the 17 of early disks. Not shown on this diagram are the different data transfer rates for the various zones. Since the angular velocity of the disk is constant and there are substantially more sectors on the outer tracks than there are on the inner ones, then the data transfer rates for the outer tracks must be higher than those for the inner ones. An interesting side effect of this feature is that benchmark tests run on the disk when new, and then again after being in use for some time, might suggest that the disk is getting slower. In fact, all that is happening is that the benchmark tests are being run on unused tracks that are now closer to the centre than when the disk was new. This is because sectors are normally assigned by the file system from the outermost cylinder inwards. What the benchmark is telling us is what we already knew: the data transfer rates of the inner zones are slower than those of the outer zones.
This ZBR approach, however, now poses us a serious problem. Our use of CHS addressing has always assumed a known constant value27for sectors per track across the whole of the disk. If that assumption is now no longer true, then we would need to give a fourth parameter with each CHS address: that of the sectors per track for the particular zone within which that address resided. This would significantly complicate the disk interface.
A much better solution is for the controller topretendto us that there is no zoning and to provide to us a contrived sectors per track value, normally 63, that is constant for the whole disk. When the controller receives, from the outside world, a CHS address that is based on this contrived sectors per track value, it simplytranslates this external CHS address into its own internal zoned address. In this way, we need have no knowledge of the zoning within the disk system in order to address it. We also need have no knowledge of any of the low-level formatting data structures, encoding methods or bad sector mapping strategies that may be used by the manufacturer, since the controller hides all of this from us. Indeed, modern
Zone 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Spt 122 135 142 153 162 170 180 185 195 205 214 225 225 229 232 Tracks 454 454 454 454 454 454 454 454 454 454 454 454 454 454 454
Fig. 5.10 Zoned bit recording.
24 It is not a requirement that zones be of equal numbers of tracks. This disk just happens to be built this way.
25 Just a reminder that tracks in this context is synonymous with cylinders. 26 Note that the figure of 225 for both zones 2 and 3 is correct.
27 If in doubt about this, consider repeatedly incrementing the sector count by 1 and then trying to decide when the sector count should revert to 1 and the head count should be incremented. The CHS address system only works for a fixed sector per track value across the disk.
controllers constantly monitor the internal state of the disk, using spare capacity as we have described above to re-map sectors and tracks as defects are detected, so that the disk appears “perfect” for most of its life.
Such a re-mapping strategy has important implications for forensic computing analysts. If the disk has been used to hold unlawful material, it is possible that some of the sectors holding that unlawful material might have become re-mapped inter- nally by the disk controller following the detection of defects. Under these circum- stances, the material from the bad sectors would automatically be duplicated and held in the re-mapped good sectors elsewhere. To the external world, only the good sectors would be apparent or accessible, but the disk would still contain copies in the bad sectors that were now inaccessible. This means that even the most compre- hensive of normal disk wipes would not remove the bad sector copies, since they are no longer accessible outside of the controller. Although these re-mapped bad sectors might be technically quite difficult to access, knowledge of the manufacturer’s controller program and, in particular, the factory mode special command set for the controller, might permit some of the material to be restored. The implications of this are that completely wiping a ZBR disk using normal processes will not guarantee the removal of all unlawful material, and therefore a hard disk which had contained such material should not be returned to a defendant. This was the ruling given in a Manchester Crown Court judgement in 2002 (R v Aslett, 2002).
However, it should be noted that later versions of the ATA specifications, which we discuss in a subsequent section, include an optional security feature, which is described as follows: “When normal erase mode is selected, the SECURITY ERASE UNIT command writes binary zeroes to all user data areas. The enhanced erase mode is optional. When enhanced erase mode is selected, the device writes predetermined data patterns to all user data areas. In enhanced mode, all previously written user data is overwritten, including sectors that are no longer in use due to reallocation” (McLean, 2001, p. 231). This implies that all disks conforming to this ATA specifi- cation can be expected to include commands for a “normal erase mode”. This mode enables complete wiping, by the disk controller, of the user area, but does not include wiping of the re-mapped bad sectors. It further implies that disks conforming to this ATA specificationoptionallymay include commands for an “enhanced erase mode”. This mode enables complete wiping, by the disk controller, of the user area and does include wiping of the re-mapped bad sectors. However, there is no guarantee currently that any particular disk will support this optional feature.
Finally, to conclude this section, it should be noted that theinternal sector trans- lationthat is carried out by the disk controller to convert those outside world CHS addresses that use a contrived sectors per track value to its own internal zoned address form is not to be confused with theCHS address translationissues that we are going to be discussing shortly in connection with the various legacy interface problems.