Operating Systems 2015F Lecture 9

Lecture 9
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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