Soma at 20:16, 7 October 2015

2015-10-07T20:16:52Z

← Older revision		Revision as of 20:16, 7 October 2015
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			==Video==

			The video from the lecture given on October 7, 2015 [http://homeostasis.scs.carleton.ca/~soma/os-2015f/lectures/comp3000-2015f-lec09-07Oct2015.mp4 is now available].


			==Notes==

	<pre>		<pre>
	Lecture 9		Lecture 9

Soma: Created page with "

 Lecture 9 ---------  Beyond filesystems (& better filesystems)  - what happens when they get corrupted?  - what happens when we need to recover old data  - what about mu..."

2015-10-07T18:20:09Z

Created page with "<pre> Lecture 9 --------- Beyond filesystems (& better filesystems) - what happens when they get corrupted? - what happens when we need to recover old data - what about mu..."

New page

<pre>
Lecture 9
---------

Beyond filesystems (& better filesystems)
- what happens when they get corrupted?
- what happens when we need to recover old data
- what about multiple storage devices?

Journaled filesystems
- avoid long fsck's
- strategy: write everything TWICE
- write once sequentially, the second time randomly
- first write pass is to a log called the journal: sequential data structure of updates to FS
- example: changed inode 2203, updated data block 18523, blah
- to get a full view of the FS, you have to check the journal and the regular fs
- limit the size of the journal and periodically commit its changes to the rest of the FS
- why it helps: corruption happens from interrupted writes

- with this, an fsck is just playing back the journal

Journaled filesystems work well when workload is mostly reads. But what if they are mostly writes?
- bad because we're writing twice, so half (or much less) performance)
- but what if we only wrote once to the log?
=> log-structured filesystems

log-structured filesystems have a nice feature:
- they don't write to the same block too often
- writing to the same block with flash storage too many times is BAD, destroys the block

- flash firmware implements a log-structured like "filesystem" (layer)

Logical Volume Management (LVM)
- filesystem just needs an array of blocks
- why not merge arrays of blocks from multiple devices?

Modern filesystems is to combine LVM and regular FS
</pre>

Operating Systems 2015F Lecture 9 - Revision history

Soma at 20:16, 7 October 2015

Soma: Created page with "
Lecture 9 --------- Beyond filesystems (& better filesystems) - what happens when they get corrupted? - what happens when we need to recover old data - what about mu..."