Hardware Reference
In-Depth Information
These controllers created 512-byte sectors that consumed 587 total bytes on the disk, for an improved
sector efficiency of 87.2%. After 1987, the AT disk controller became part of the drive, which is
what we call the ATA interface, often also referred to as IDE (Integrated Drive Electronics).
Integrated controllers allowed much tighter signal timing and speed, allowing for more sectors to be
stored on each track and for additional improvements in sector efficiency.
To further reduce sector overhead, IBM pioneered an innovation called the No-ID sector format,
which was first seen in IBM's DSAA family drives in the early 1990s. Using this technology, the ID
information was removed from the sector header and instead was determined via other means. To
determine sector IDs without the ID information in the header, cylinder information was placed in the
embedded servo data already on the disk, which was combined with stricter timing controls and a
defect map loaded from the drive into the disk controller RAM on startup. Using the No-ID format,
the drive was able to determine sector IDs without having the ID explicitly written in the sector
header, resulting in a dramatic improvement in sector efficiency.
Table 9.4
shows the format of a 512-byte sector on a modern drive.
Table 9.4. 512-Byte Sector Format in a Modern Drive
In a modern 512-byte sector format, you can see that the header has been reduced to only 15 bytes, a
dramatic reduction from the first PC hard drives where it consumed 89 bytes. But even with that
dramatic reduction in the size of the header, the sector on a modern drive still takes up 577 bytes on
the disk, resulting in a sector efficiency of 88.7%. Note that although this is typical for a modern
drive, the actual specific number of additional bytes required for the sector header and trailer (and
therefore the overall efficiency) can vary somewhat from these figures depending on the drive/media
type and format.
What is surprising is that while the header has been significantly reduced in size, the trailer
containing the ECC information has expanded dramatically such that the overall sector efficiency has
only improved by about 1.5% since 1984! This is obviously due to the increase in the size of the ECC
information.
ECC
The number of ECC bytes stored (and therefore the size of the sector trailer) has always been a
compromise. More ECC bytes means the ability to correct more errors, but it also consumes more
space on the disk. The 4 ECC bytes used by the earliest controllers was sufficient to ensure an
unrecoverable read error rate of less than 1 in 10
12
(1 trillion) bits, which is equal to 125GB worth of
data. An unrecoverable error is one where the drive cannot read the data properly, even after
