1. Deciding When to Forget in the Elephant File System
By Douglas S. Santry, Michael J. Feeley, Norman C. Hutchinson, Alistair C. Veitch, Ross W. Carton, and Jacob Or
Presented By Jon LeVitre
CS 533 Concepts of Operating Systems March 14, 2006
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2. Overview Background The Elephant File System Summary
Goals and Ideas
Design
Policies
Implementation
Performance
Summary
CS 533 Concepts of Operating Systems March 14, 2006
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3. Background Disk space is becoming exponentially cheaper and larger
Human productivity remains constant
Data is protected from most forms of failure (except user error)
It's a good time to consider ways to make file systems protect users from themselves
CS 533 Concepts of Operating Systems March 14, 2006
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4. Previous Work
Cedar used copy on write to automatically retain recent versions
Plan-9, AFS, and WAFL use checkpointing
Applications maintain document history
Trashcan prevents accidental deletion
Users make their own copies
CS 533 Concepts of Operating Systems March 14, 2006
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5. Goals of EFS
Give users the ability to undo recent changes (both writes and deletes)
To save storage space, long term history only has important versions
CS 533 Concepts of Operating Systems March 14, 2006
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6. Observations
The user's ability to recognize crucial file versions deteriorates over time
Any solution that relies solely on the user to identify landmark versions is problematic (but they need to be allowed to do it)
CS 533 Concepts of Operating Systems March 14, 2006
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7. Types of Files Read-only Derived Cached Temporary* User-modified*
* Only these need protection
CS 533 Concepts of Operating Systems March 14, 2006
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8. General Design Logical file deletion
Copy-on-write (version becomes official when file is closed)
File versions are named by combining pathname, date, and time (not unique)
Retention policy specified by file (or group of files)
File system cleaner reclaims blocks
CS 533 Concepts of Operating Systems March 14, 2006
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9. File Retention Policies
Keep One
Keep Save
Recent changes only
Keep Landmarks
Heuristic: keep long-lived versions
Also let users specify landmarks
Keep All
CS 533 Concepts of Operating Systems March 14, 2006
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10. File Implementation The inumber points to an imap
Temperature is a heuristic used by the cleaner
For non-versioned files, imap points to an inode
Otherwise, imap points to an inode log
CS 533 Concepts of Operating Systems March 14, 2006
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11. Directory Implementation
Directories map names to inumbers
Directories store versioning information explicitly
Each directory entry stores the creation time and delete time (if any)
Entries for deleted files can be moved to a history inode
CS 533 Concepts of Operating Systems March 14, 2006
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12. Microbenchmark Results
Files with Keep One Policy was about the same as FFS (write had a bug)
Files with Keep All policy
Slightly slower open, write, and close
Much slower creation
Much faster deletion
CS 533 Concepts of Operating Systems March 14, 2006
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13. More Microbenchmarks
Andrew file system benchmark (creates directory hierarchy, copies 70 source files totaling 200KB files, traverses directories, and opens and reads each file)
EFS was ~5% slower (19 seconds vs 18 seconds)
Much more file meta data:
FFS used 18KB for inodes
EFS used 444KB for inode logs
For larger test, EFS was twice as fast as FFS
CS 533 Concepts of Operating Systems March 14, 2006
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14. File Profiles
Keep One: 33.6% of files, 56.3% of bytes (98.7% of bytes written)
Keep Safe: 3.9% of files, 28.5% of bytes (only 0.6% of bytes written)
Keep Landmark: 62.4% of, 15.2% of bytes (only 0.7% of bytes written)
CS 533 Concepts of Operating Systems March 14, 2006
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15. Impact on Buffer-Cache?
Buffer-Caches reduce the number of writes to disk to improve performance
Elephant must write to disk when file is closed, even if ...
There is a write shortly after close
The file is deleted right after close.
This should be rare, so the impact should be minimal
CS 533 Concepts of Operating Systems March 14, 2006
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16. Summary Performance similar to FFS Only a few files need versioning
Robustness verified using NFS shadowing
“....we believe that the extra storage and disk write overhead incurred by using a file system such as Elephant is of minimal cost compared to the convenience and time gains... made possible”
More research was needed
CS 533 Concepts of Operating Systems March 14, 2006
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