COMP 3430 – Operating Systems
RAID and file system consistency
March 23rd, 2022
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By the end of today’s lecture, you should be able to:
▶ Choose an appropriate RAID-level given requirements.
▶ Describe what a human operator might see in files when a file system is in an inconsistent state.
The Woggle-Bug Book (Public Domain)
Let’s take a look at the course schedule.
What kind of RAID do I want?
Let’s look at some scenarios and decide what RAID level would be appropriate for the problem, situation, or person.
Pixabay License
Scenario 1: the data hoarder
Probably RAID 0.
Scenarios 2 and 3: the rest
Let’s look at both scenarios 2 and 3 (the gamer and the scientist). Which RAID level would best suit this workload?
▶ RAID 4/5
▶ Something else (what?)
Inconsistent file systems
File systems can become inconsistent in creative ways.
I think it’s safe to say this file system is inconsistent.
(Pixabay License)
A teeny tiny file system (OSTEP Chapter 42) Let’s look at extending a file (adding a new block)
▶ Update the inode (1 new direct pointer).
▶ Update the data bitmap (we’re using a block). ▶ Update the data region (for the actual data).
Four options
Four results can happen:
1. The file looks corrupted.
2. There’s less available space on the volume. 3. The file’s size is bigger.
4. No apparent difference.
One or more of these results can happen.
Corrupted looking file.
Less available space on volume.
Bigger file size.
Let’s do the quiz all together.
▶ Different RAID-levels serve different needs.
▶ Inconsistent file systems look different to regular humans. ▶ Different for each kind of failure scenario.
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