Slideshow:
Review: a disk block has
a specific format
Review: a disk block has
a specific format
Records has a specific format too !!!
The complete record structure
Information stored in the record header (in general):
Record header for fixed length records
We will consider an example next
Example record format (with padding)
Padded record:
Record header information describing the record format
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A macro view: how (record header + record data)
are stored inside a disk block
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- Fact:
- Not only does
data blocks has
a structure,
but records (stored inside
a block)
also have a
structure (format):
- Not only does
data blocks has
a structure,
but records (stored inside
a block)
also have a
structure (format):
- The
format
of a record
that is stored inside
a disk block:
- A record consists of
- A record header
- The record fields
Graphically:
- A record consists of
- The record header:
- Record header contains
information about the
fields in the
record
E.g.:
- Length
(# bytes) of the record
- For each field:
- Field name
- Offset in the record
- Time that the
record was
last updated
- And so on.... (other info)
- Length
(# bytes) of the record
- Record header contains
information about the
fields in the
record
- Record with
only
fixed length fields will have:
- The same record length for each record
- The same offset for each data field in the record
Result:
- We only need to
store:
- Record length
- Offsets of each data field
only once
- Suppose a
record contains
these field:
Name: 30 characters - must start at an address that is divisible by 4 Addr: 256 characters - must start at an address that is divisible by 4 Gender: 1 character - must start at an address that is divisible by 4 BirthDay 10 characters - must start at an address that is divisible by 4 - Alignment requirement:
- Stored
record format:
- The above
record format is
stored inside
a block
(which has a
block header):