26. RAID Flashcards
Describe workshop papers.
Short (5-6 pages), usually containing only a provocative argument, system design, or very preliminary results
Ex: “Hierarchical Filesystems Are Dead”
Describe conference papers
Long (12-14 pages), enough space to describe and evaluate a complete novel system
Example: The Scalable Commutativity Rule
Describe journal papers
Longer (>14 pages), usually a published conference paper with extra material (frequently all of the unnecessary results that they removed to make the conference page limit)
Advice: read the conference paper
What are the 5 kinds of papers discussed in class?
- (Big) idea papers
- Problem papers
- Data papers
- New technology papers
- Wrong way papers
What do (big) idea papers cover?
Presents a new approach to an existing problem or a new idea about how to build systems.
Should convince you that the solution is (1) new, (2) works, and (3) is useful
What do problem papers cover?
Presents a new problem and, usually, some ideas about how to solve it.
Should convince you that the problem is (1) new, (2) matters, and (3) that there are some ways to solve it
What do data papers cover?
Presents novel analysis or analysis of a novel data set that produces interesting insights.
Should convince you that the results are useful to the design of future systems
What do new technology papers cover?
Describe some new hardware capability or device feature and why it’s interesting.
Should convince you that the hardware can be used to build better systems
What do wrong way papers cover?
Argue that the community is solving an existing problem incorrectly. Frequently these are workshop-level papers and eventually lead to idea papers.
Should be able to convince you that everyone else is confused and misguided (Good luck!)
What does RAID stand for?
Redundant Arrays of Inexpensive Disks
What kind of paper is the RAID paper?
A big idea paper.
It spawned a commonly-used technology, an entire industry, and lots of similar approaches
What is the big idea of the RAID paper?
Several cheap things can be better than one expensive thing
Where else do we see this idea applied today?
Multicore processors, Google using thousands of Dell computers in network, and crowdsourcing
What is the problem that the RAID paper identifies?
Computer CPUs are getting faster, computer memory is getting faster, and hard drives are not keeping up
While we can imagine improvements in software file systems via buffering for near term I/O demands, we need innovation to avoid an I/O crisis
What is the problem with the RAID solution to the cost of SLEDs?
Many cheap things fail much more frequently than one expensive thing.
This means there needs to be a plan in place to handle failures
How does RAID 1 work?
RAID 1 is mirroring.
There are two duplicate disks.
Writes must go to both disks, reads can come from either.
Performance: better for reads.
Capacity: Unchanged
How does RAID 2 work?
Byte-level striping, single error disk. (meaning data written to disk is spread out byte by byte across multiple disks)
Hamming codes are used to detect failures and correct errors
Most reads and writes require all disks
Capacity: “improved”
How does RAID 3 work?
RAID 3 only corrects errors, since disks can detect when they fail.
Byte-level striping, single parity disk. (for each byte, there’s an extra bit of information stored somewhere on another disk)
Most reads and writes require all disks.
Capacity: “improved”
How does RAID 4 work?
Block-level striping, single parity disk.
Better distribution of reads between disks due to larger stripe size, but all writes must access the parity disk.
Performance: improved for reads
How does RAID 5 work?
Block-level striping.
Multiple parity disks.
Better distribution of writes between disks.
Performance: improved for writes
What is RAID 0?
Each disk stores half of the data
No error correction or redundancy
Performance: fantastic!
Capacity: fantastic!
Redundancy: ZERO!
What is RAID 0?
RAID 0 is not RAID.
Each disk stores half of the data (2 disks total)
No error correction or redundancy
Performance: fantastic!
Capacity: fantastic!
Redundancy: ZERO!
What is block-level striping?
Writes to disk writes blocks to the many disks in RAID one block at a time, so that a multi-block write happens across multiple disks.
What is block-level striping?
Writes to disk writes blocks to the many disks in RAID one block at a time, so that a multi-block write happens across multiple disks.
How many failures can RAID arrays tolerate?
One (or more) disks. The more failures you want your RAID arrays to handle, the more disk space you need to set aside for keeping track of failures (thus reducing your disk’s capacity).
The challenge is identifying the optimal balance between failure tolerance and disk capacity.
What two things happen when a disk fails in a RAID system?
- Once one (or several) disks fail, the array is vulnerable to data loss.
- An administrator must replace the disk(s) and then rebuild the array
