Digital Equipment RW504-ZA, Service Manual

Page: 168 / 220

6.5.6 Slip Sparing Algorithm
The slip sparing algorithm maps logical blocks to physical blocks to avoid
defective sectors found during a certification process. The list of defective sectors
is maintained in the Primary Defect List (PDL). During an address translation,
the logical blocks are ‘‘slipped’’ past any defective sectors, thus the name ‘‘slip
sparing.’’ As an example, say there are defective sectors at block addresses 20
and 30, and you want the physical address of logical block 40. Since addresses
20 and 30 are defective they should be slipped past, so logical block address 20 is
now physical block address 21, and logical block address 30 is now physical block
address 32, taking into account both physical blocks 20 and 30 being slipped past.
This would result in physical block address 42 being the translation for logical
block address 40.
Note that this is not a truly accurate example for three reasons:
1. PDL entries are given in track/sector form, not as block addresses. The final
translated address must also be in track/sector form.
2. There is a 3-track offset added to the physical block address, 51 sectors for
1024 bytes/sector media and 93 sectors for 512 bytes/sector media.
3. This example does not take into account the effects of Spare Groups preceding
this sector. The User and Spare Groups are determined after slip sparing, in
a type of intermediate block addressing.
Slip sparing is always the first step of address translation for 130 mm optical
disks, followed by User and Spare Grouping, and replacement sparing. The data
structures for slip sparing and User and Spare Grouping (the PDL and DDS
respectively) are created or updated only during a certification/format process,
such as during a SCSI Format Unit Command. After certification, any additional
defect management updating is done through the replacement sparing algorithm.
6.5.7 Replacement Sparing Algorithm
Once a disk has been certified and is being used by a customer, additional
defective sectors may develop. The replacement sparing algorithm is intended to
manage these defective sectors.
As was mentioned earlier, the DDS allows for a number of sectors to be reserved
for future use by the replacement sparing algorithm. These ‘‘spare sectors’’ reside
in the Spare Groups, and are referred to via entries in the SDL. Each SDL entry
consists of a defect and its replacement pair. The defect is always a sector in
a User Group, and the replacement is a sector from a Spare Group. Both are
given in track/sector form. The SDL can contain up to 2048 entries. Again, the
maximum number of total defective sectors in the PDL and SDL combined is
2048.
6–20 Theory of Operation