Lecture 12 File systems

Lecture 12 File systems - File system fun File systems =...

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Unformatted text preview: File system fun File systems = the hardest part of OS- More papers on FSes than any other single topic Main tasks of file system:- Dont go away (ever)- Associate bytes with name (files)- Associate names with each other (directories)- Can implement file systems on disk, over network, in memory, in non-volatile ram (NVRAM), on tape, w/ paper.- Well focus on disk and generalize later Today: files, directories, and a bit of performance 1/38 The medium is the message Disk = First thing weve seen that doesnt go away- So: Where all important state ultimately resides Slow (ms access vs ns for memory) Huge (1001,000x bigger than memory)- How to organize large collection of ad hoc information?- Taxonomies! (Basically FS = general way to make these) 2/38 Disk vs. Memory MLC NAND Disk Flash DRAM Smallest write sector sector byte Atomic write sector sector byte/word Random read 8 ms 75 s 50 ns Random write 8 ms 300 s* 50 ns Sequential read 100 MB/s 250 MB/s > 1 GB/s Sequential write 100 MB/s 170 MB/s* > 1 GB/s Cost $.081/GB $3/GB $10-25/GB Persistence Non-volatile Non-volatile Volatile *Flash write performance degrades over time 3/38 Disk review Disk reads/writes in terms of sectors, not bytes- Read/write single sector or adjacent groups How to write a single byte? Read-modify-write- Read in sector containing the byte- Modify that byte- Write entire sector back to disk- Key: if cached, dont need to read in Sector = unit of atomicity.- Sector write done completely, even if crash in middle (disk saves up enough momentum to complete) Larger atomic units have to be synthesized by OS 4/38 Some useful trends Disk bandwidth and cost/bit improving exponentially- Similar to CPU speed, memory size, etc. Seek time and rotational delay improving very slowly- Why? require moving physical object (disk arm) Disk accesses a huge system bottleneck & getting worse- Bandwidth increase lets system (pre-)fetch large chunks for about the same cost as small chunk.- Trade bandwidth for latency if you can get lots of related stuff.- How to get related stuff? Cluster together on disk Memory size increasing faster than typical workload size- More and more of workload fits in file cache- Disk traffic changes: mostly writes and new data 5/38 Files: named bytes on disk File abstraction:- Users view: named sequence of bytes- FSs view: collection of disk blocks- File systems job: translate name & offset to disk blocks: { file, offset } FS disk address File operations:- Create a file, delete a file- Read from file, write to file Want: operations to have as few disk accesses as possible & have minimal space overhead 6/38 Whats hard about grouping blocks?...
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This note was uploaded on 03/13/2010 for the course CS 02523 taught by Professor Davidmieres during the Winter '10 term at A.T. Still University.

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Lecture 12 File systems - File system fun File systems =...

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