Undo And Redo Rules

Submitted By Tomas-Amador
Words: 1334
Pages: 6
Open Document
[Step (3) of Restart] At this point, x’s last committed value is only restored in the cache. The CM will restore it in the stable database when it replaces x’s cache slot.
H. [Step (3) of Restart] The before image of x wrt T, can be found in the log.
I. [Step (4) of Restart] Upon recovery from a system failure, the scheduler must wait for the acknowledgment that Restart has been completed by the RM. It may then start sending operations to the RM again.
Undo and Redo Rules
This algorithm satisfies the Undo Rule. Suppose the location of x in the stable database contains the last committed value of X, say V, written by transaction
T,. When T, wrote into x, the RM inserted [T;, X, V] in the log (see step (2) of
RM-Write). By the Garbage Collection Rule, since v is the last committed value of X, this record cannot have been removed. In particular, it will still be in the log when the CM overwrites v in the stable database, as desired for the
Undo Rule.
The Redo Rule is likewise satisfied. All of a transaction’s updates are recorded in the log before the transaction commits, whether or not they were also recorded in the stable database. By the Garbage Collection Rule they must still be there when the transaction commits.
Since the algorithm satisfies the Undo and Redo Rules, Restart can always find in stable storage the information that it needs for restoring the last committed value of each data item in the stable database. In step (3), Restart redoes an update to x by a committed transaction, or undoes an update to x by an active or aborted transaction, only if no other committed transaction subsequently updated x. Thus, when Restart terminates, each data item will contain its last committed value. Moreover, Restart is idempotent. If it is interrupted by a system failure and reexecutes from the beginning, the updates it redoes and undoes in step (3) are the same as those it would have done if its first execution had not been interrupted.
Checkpointing
The Restart procedure just sketched may have to examine every record ever written in the log - except, of course, those that have been garbage collected,
This is still a very large number of records, since garbage collection is an expensive activity and is carried out fairly infrequently. Moreover, since most data items in the database probably contain their last committed values at the time of the failure, Restart is doing much more work than necessary This inefficiency of Restart is an important issue, because after a system failure, the
DBS is unavailable to users until Restart has finished its job.
184 CHAPTER 6 I CENTRALIZED RECOVERY
This problem is solved by the use of checkpointing methods. In general, checkpointing is an activity that writes information to stable storage during normal operation in order to reduce the amount of work Restart has to do after a failure.
Checkpointing performs its work by a combination of two types of updates to stable storage: (1) marking the log, commit list, and abort list to indicate which updates are already written or undone in the stable database, and (2) writing the after images of committed updates or before images of aborted updates in the stable database. Technique (1) tells Restart which updates don’t have to be undone or redone again. Technique (2) reduces the amount of work that Restart has to do by doing that work during checkpointing.
Technique (1) is essential to any checkpointing activity. Technique (2) is optional. One simple checkpointing scheme is periodically to stop processing transactions, wait for all active transactions to commit or abort, flush all dirty cache slots, and then mark the end of the log to indicate that the checkpointing activity took place. This is caIled commit consistent checkpointing, because the stable database now contains the last committed value of each data item relative to the transactions whose activity is recorded in the log. With commit
Show More

Related Documents: Undo And Redo Rules

My Rhetorical Analysis

or when I was scared to turn in my papers, but here I am writing my last paper for the class. I made it and I succeeded. I have found pride in a few of my papers, but I also know what I can work on. I have realized what works for me and what doesn’t. This class has showed me many of my strengths and my weaknesses. I feel that this class has really helped me to grow as a writer and as a learner. My Rhetorical analysis was the piece of writing I was most proud of this semester. Eight hours and three…

Words 1064 - Pages 5

Group Presentation Analysis

In this class we have been required to present many presentations on a variety of information. With all of these presentations I have noticed what I excel in with presentations and what I lack in. What I lack in while I present is maintaining eye contact with the audience. This was very present with the last group presentation where I didn't know how to pronounce russian names and it made me nervous and I looked everywhere but my audience. Another thing that I lack in with my presentation skills…

Words 938 - Pages 4

Research And Academic Writing

Final My growth in this class as a writer is day and night. Coming over pretty confident from E 105, I really did not know what to expect from the class and myself. There has been a tremendous amount of changes in my writing. I have become more of a college style writer and this class pointed out my weaknesses as a writer. I was able to zone in on these weaknesses and improve as an academic writer. Also, I learned the importance of drafting and why it is so important for the finished product. In…

Words 1366 - Pages 6

Reflection: Writing and Articles Essay

ways one can learn from their own writing through revising their own paper. I believe I have improved on my writing by revision. A good example that I can reflect on is my paper on An Analysis of Two Articles on The Final Moments of King Richard III. One can see where there needs to be more attention on making clear points by using increased examples to help one understand as well as using more distinctive words. In my analysis, on the two articles on The Final Moments of King Richard III…

Words 992 - Pages 4

Rhetorical Analysis Of Dual Credit Composition

express my creative side that I was not aware I had. Throughout elementary school, I always had a knack for creating stories, but I never thought that it would carry into my older years. During American Studies, my junior year, I lacked the motivation that I had this year, to write papers. My senior year brought a new person who enjoyed writing papers more than she ever did. I started to look at writing papers as more than just meeting the deadlines with good enough writing. Writing papers started…

Words 771 - Pages 4

Personal Narrative: Improving My Writing Skills

As I reapproached school in the second semester I realized I needed to focus o my writing. I did not do well in my writing during UNV-101, so I decided this time I would improve myself over the course of our 3 essays. I need to have a good set of writing skills for when I become a teacher so I can grade effectively and correctly. Welcome to the Matrix has helped me improve my writing skills drastically over the course of the semester. In the beginning of the semester, we started off with a analysis…

Words 1085 - Pages 5

Research Paper On Organic Food

to understand and get myself adapted to the system. When I gave a kick start to my curriculum at SNHU, no offence, it was a fantabulous experience. When I actually started working on my research paper, in a very less time, I had to analyze persistence factors which are applicable and ideal to research paper. In my research paper, I learnt many things like in text citations, annotations, bibliography. In research paper, we need to keep in text citations for each and every paragraph that means any information…

Words 574 - Pages 3

My Rhetorical Analysis

I have grown as a writer from my Rhetorical Analysis’s advisory draft. My confidence has grown as a writer from the start of this class. I know that my first writing will not be the best, but trough revision my writing will get better. One way that I have grown is that I can pull better information out of my sources and support more effectively. When I was writing my Rhetorical Analysis, I did not effectively support my evidence, especially with my logos. I am still not fully confident writing about…

Words 652 - Pages 3

Peer Review and Writing Skills Essay

During my WRIT 102 class, I have grown to be a strong, consistent writer, and I know that my papers have seriously improved since even last semester. Now although there is no one way to write well, I believe my essays have become more articulate, interesting, and easier to understand. Not only have I improved my papers in WRIT 102, but my writing skills have helped me prepare speeches for my Speech 102 class. There are three ways in which I was able to improve in, including proofreading, peer…

Words 754 - Pages 4

Essay on Self Evaluation

Evaluating My Writing Skills When I first go about writing a paper, I tend to brainstorm about the topic specifically. I like to think about the details of my topic and come up with supporting facts or evidence for my papers. My mental state while writing is having a focused mind set. For me, it is almost impossible for me to do anything productive while writing my papers if I’m listening to music or talking to other people. I need to have little to no distractions when trying to complete my papers. I…

Words 474 - Pages 2