Please answer the following questions briefly:

a.	What are the properties required of LSNs?
b.	What are the fields in an update log record?
c.	What are redoable log records?
d.	What are the differences between update log records and CLRs?

Hello Eksolutions,

a. LSN's (or Log Sequence Numbers) must be
 - unique
 - always increasing

b. Fields in an update log record includes:
 - PrevLSN
 - Transaction Id (or XID)
 - Type (update)
 - pageID
 - length
 - offset
 - before-image
 - after-image

c. Redoable log records generally refer to actions that must be
"redone" as part of crash recovery. Starting at page 18 of the first
reference below there is a good illustration and explanation of the
difference between redoable and undone updates and how they are used
in crash recovery.

d. Update log records are as described above. A Compensation Log
Record (or CLR) records "Undo" actions and are never "undone". You
write a CLR before you restore the old value of a page (and thus you
log even in crash recovery). It generally has an additional field -
"undo next LSN" to refer to the next LSN to be undone.

If the acronyms are not correct - please use a clarification request
for get a better answer.

For reference material, search with phrases such as
  database LSN CLR update redoable log records

Material for this answer was drawn primarily from
  http://www.cs.colorado.edu/~rick/Classes/3287/Lectures/lec8_bw.pdf
[a PDF of a powerpoint presentation]
and
  http://www.cs.ndsu.nodak.edu/~vshi/cs366/Lectures/ch20.html

  --Maniac