Unix file system snapshots

• Describes all files on file system.
• Daily for one year

NFS traces

• All NFS requests to large file server.
• Continuous for two weeks.

Generating Aging Workload

Populate file system.

• Create files present in first snapshot.

Add inter-day file activity.

• Compare successive snapshots.
• Identify created and deleted files.
• Add corresponding create, write, and delete operations.

Generating Aging Workload

Add intra-day file activity.

• Use NFS traces to model short-lived file activity.
• Intersperse create, write, and delete operations based on model.

Sample Workload

• Aging Workload:
• Seven months of activity
• 1 GB file system
• ~1.3 million file operations
• Writes 87.3 GB to disk
• Typical run time is 39 hours.

Verifying Workload

• Start with empty file system.
• Age file system using workload.
• Execute file operations from workload on the test file system.
• Compare file fragmentation on aged file system to last snapshot of file system from which workload was generated.

Verification Metric

• Layout Score
• Measures quality of file layout
• Range: 0.0 - 1.0
• Inversely proportional to file fragmentation
• Score is percentage of file system blocks that are contiguous
• 1.0 => All files are contiguously allocated
• 0.0 => No contiguous allocation

Aging Verification

Example

• Modification to UNIX file system (FFS)
• Use aging to evaluate performance trade-offs.

Test Platform

• 200 MHz Pentium Pro
• 32 MB RAM
• PCI Bus
• NCR 53c825 SCSI controller
• Fujitsu M2694ES disk
• 1 GB, 5400 RPM, 15 Heads, 94 Sect./Track (avg.), 1818 Cyl. 9.5 ms Avg. Seek
• BSD/OS 2.1
• 8 KB file system block size
• maxcontig = 7 blocks (56 KB)

Baseline FFS Performance

(Aged file system)

The UNIX File System (FFS)

Cylinder Groups

• Cylinder groups are allocation pools.
• They exploit locality of reference.
• Related data are collocated in same cylinder group.
• All files in a directory
• Sequential blocks of a file

File Allocation

• First 12 file data blocks are allocated from same cylinder group as the file's directory.
• The 13th and subsequent blocks are allocated in a different cylinder group.
• All files have a large seek between 12th and 13th block.
• 12 blocks = 96 KB

Solution

• NoSwitch file system
• Don't switch cylinder groups after the 12th file block.

Potential Problem

• Too many large files in one directory would cause cylinder group to run out of space.
• Creates split files.
• Files in different cylinder group than their directory.
• Extra seek to get from directory to file.
• But does this happen?
• If so, how does it affect performance?

Evaluation of NoSwitch

• Age two file systems, one that switches cylinder groups, and one that doesn't
• Compare the resulting file systems
• Overall performance
• Number of split files.

Performance

Number of Split Files

	Baseline	NoSwitch
Number of Files	33,797	33,797
Number of Split Files	4,312	9,155
Percentage of Split Files	13%	27%

Hot File Benchmark

• Measure performance using files from aging workload
• Files modified during final 30 days
• 92 MB (14.5% of allocated storage)
• 3,207 files (9.5% of files)
• 119 files large enough to benefit from NoSwitch
• Two phase benchmark:

Overwrite entire file set

Hot File Performance

	Baseline	NoSwitch
Layout Score	0.928	0.931
Number of Split Files	327	594
Read Throughput	0.81 MB/sec	0.84 MB/sec
Write Throughput	0.49 MB/sec	0.50 MB/sec

Analysis

• NoSwitch file system improves performance of medium and large files.
• NoSwitch file system increases the number of split files.
• Net effect is small performance improvement.
• Exact trade-off depends on workload!

Conclusions

• Benchmarking empty file systems is unrealistic.
• Benchmarking empty file systems can be misleading.
• File system aging is a technique for increasing the relevance of file system benchmarking.

 

 

Don't benchmark empty
file systems!

File System Aging:
Increasing the Relevance of
File System Benchmarks

 

Keith A. Smith
Margo I. Seltzer

keith@eecs.harvard.edu
margo@eecs.harvard.edu

http://www.eecs.harvard.edu/~keith/sigmetrics97

Fragmentation Metric

• Layout Score measures fragmentation
• Fraction of blocks that are contiguous
• Ignores first block of a file.

Sequential I/O Benchmark

• 32 MB data set
• Uniform file size (16 - 16,384 KB)
• 25 files per directory
• Two Phases
• Create Phase: Create and write all files
• Read Phase: Read all files

Comparison (empty)

Comparison (aged)

Aging Verification

Performance (empty)

Seek Distances in Split Files

Future Work

• Improve aging algorithm
• Expand to cover more workloads.
• Parameterize for amount of aging or size of file system.