multi-pack-index Git Command Guide

The git multi-pack-index command writes and verifies multi-pack-index (MIDX) files that optimize access to multiple pack files by providing a unified index across all pack files in a repository.

git multi-pack-index Syntax:

git multi-pack-index [--object-dir=<dir>] [--[no-]bitmap] <sub-command>

Global Options:

Option	Description
--object-dir=<dir>	Use specified directory for Git objects
--[no-]bitmap	Control bitmap generation for multi-pack-index
--[no-]progress	Show/hide progress indicators

Sub-commands:

Index Management:

Sub-command	Description
write	Write a new multi-pack-index file
verify	Verify integrity of multi-pack-index file
expire	Delete expired multi-pack-index files
repack	Repack pack files using multi-pack-index

Information Commands:

Sub-command	Description
show	Show information about multi-pack-index
read	Read and display multi-pack-index contents

Understanding Multi-Pack-Index:

Pack File Architecture:

Traditional:  pack-1.idx → pack-1.pack
              pack-2.idx → pack-2.pack
              pack-3.idx → pack-3.pack

Multi-Pack:  multi-pack-index → pack-1.pack, pack-2.pack, pack-3.pack

MIDX Benefits:

Performance: Single index lookup across multiple packs
Storage:    Reduced index overhead for many small packs
Memory:     Lower memory usage for pack access
Network:    Optimized for partial clone operations

MIDX File Structure:

multi-pack-index file contains:
- Header with format version and hash type
- Pack file list with offsets and checksums
- Object index sorted by SHA-1
- Optional bitmap for reachability queries
- Trailer with checksum

Basic Usage Examples:

Writing Multi-Pack-Index:

# Write multi-pack-index for current repository
git multi-pack-index write

# Write with progress indication
git multi-pack-index --progress write

# Write for specific object directory
git multi-pack-index --object-dir=/path/to/objects write

# Write with bitmap for performance
git multi-pack-index --bitmap write

Verifying Multi-Pack-Index:

# Verify multi-pack-index integrity
git multi-pack-index verify

# Verify with detailed output
git multi-pack-index --progress verify

# Verify specific object directory
git multi-pack-index --object-dir=/cache/objects verify

Showing Index Information:

# Show multi-pack-index details
git multi-pack-index show

# Display pack file list
git multi-pack-index show | head -20

# Show index statistics
git multi-pack-index show | grep -E "(objects|packs|size)"

Expiring Old Indexes:

# Remove expired multi-pack-index files
git multi-pack-index expire

# Expire with force (remove all)
git multi-pack-index expire --force

# Expire indexes older than specific time
git multi-pack-index expire --expire=2.weeks.ago

Advanced Multi-Pack-Index Operations:

Repack Optimization:

# Repack using multi-pack-index information
git multi-pack-index repack

# Repack with specific batch size
git multi-pack-index repack --batch-size=50

# Repack to specific pack size limit
git multi-pack-index repack --max-pack-size=100m

Bitmap Operations:

# Create multi-pack-index with bitmap
git multi-pack-index --bitmap write

# Verify bitmap integrity
git multi-pack-index verify --bitmap

# Use bitmap for performance analysis
git rev-list --use-bitmap-index --count HEAD

Custom Object Directory Management:

# Manage multi-pack-index for alternates
git multi-pack-index --object-dir=/shared/objects write

# Verify shared object store
git multi-pack-index --object-dir=/cache/git/objects verify

# Show shared index information
git multi-pack-index --object-dir=/alternate/path show

Batch Processing:

# Process multiple repositories
for repo in /repos/*; do
    if [ -d "$repo/.git" ]; then
        cd "$repo"
        echo "Processing $repo"
        git multi-pack-index write --quiet
        cd - >/dev/null
    fi
done

# Parallel processing for large installations
find /repos -name ".git" -type d -print0 | \
xargs -0 -n1 -P4 bash -c '
    cd "$1/.."
    git multi-pack-index write --quiet
' _

Integration with Git Workflows:

Repository Maintenance:

#!/bin/bash
# Comprehensive repository maintenance with multi-pack-index

optimize_repository() {
    echo "Optimizing repository with multi-pack-index"

    # Clean up loose objects
    git prune-packed

    # Repack existing packs
    git repack -a -d --write-bitmap-index

    # Create multi-pack-index
    git multi-pack-index write --bitmap

    # Verify optimization
    git multi-pack-index verify

    # Show space savings
    echo "Repository size after optimization:"
    du -sh .git/

    echo "Pack files: $(ls .git/objects/pack/*.pack | wc -l)"
    echo "Multi-pack-index: $(ls .git/objects/pack/multi-pack-index* 2>/dev/null | wc -l)"
}

optimize_repository

Server-Side Optimization:

# Optimize Git server repositories
server_optimization() {
    local repo_path="$1"

    cd "$repo_path"

    # Update server info
    git update-server-info

    # Create multi-pack-index for fast clones
    git multi-pack-index write --bitmap

    # Generate pack bitmap for partial clones
    git repack -a -d --write-bitmap-index

    # Verify server readiness
    git multi-pack-index verify

    echo "Server optimization complete for $repo_path"
}

# Optimize all server repositories
for repo in /srv/git/*; do
    server_optimization "$repo"
done

CI/CD Pipeline Integration:

# Use multi-pack-index in CI for faster operations
ci_optimization() {
    echo "Setting up repository for CI optimization"

    # Shallow clone with bitmap
    git clone --depth=1 --no-checkout --filter=blob:none <url> repo
    cd repo

    # Create multi-pack-index for analysis
    git multi-pack-index write --bitmap

    # Run analysis operations
    git rev-list --use-bitmap-index --count HEAD

    # Clean up for artifact storage
    git multi-pack-index expire

    echo "CI optimization complete"
}

ci_optimization

Configuration and Optimization:

Performance Tuning:

# Configure multi-pack-index behavior
git config core.multiPackIndex true                    # Enable MIDX
git config pack.writeBitmapIndex true                  # Write bitmaps
git config pack.writeReverseIndex true                 # Write reverse indexes

# Optimize for large repositories
git config pack.packSizeLimit 100m                     # Limit pack sizes
git config pack.threads 0                              # Auto-detect CPU threads

# Memory optimization
git config pack.deltaCacheSize 512m                    # Delta cache size
git config pack.windowMemory 512m                      # Window memory

MIDX Maintenance:

# Regular maintenance schedule
git config maintenance.gc.enabled true
git config maintenance.multiPackIndex.enabled true

# Custom maintenance
maintenance_multi_pack() {
    # Expire old indexes
    git multi-pack-index expire

    # Repack if needed
    if [ $(ls .git/objects/pack/*.pack 2>/dev/null | wc -l) -gt 10 ]; then
        git multi-pack-index repack --batch-size=25
    fi

    # Verify integrity
    git multi-pack-index verify --quiet
}

# Run weekly
maintenance_multi_pack

Storage Optimization:

# Analyze pack file efficiency
analyze_pack_efficiency() {
    echo "Pack file analysis:"

    # Show pack file sizes
    ls -lh .git/objects/pack/*.pack

    # Show multi-pack-index info
    git multi-pack-index show | head -10

    # Calculate efficiency
    total_objects=$(git rev-list --all --count)
    pack_objects=$(git verify-pack .git/objects/pack/*.idx 2>/dev/null | wc -l)

    echo "Total objects: $total_objects"
    echo "Packed objects: $pack_objects"
    echo "Efficiency: $((pack_objects * 100 / total_objects))%"
}

analyze_pack_efficiency

Troubleshooting Common Issues:

MIDX Creation Failures:

# Debug multi-pack-index creation
GIT_TRACE=1 git multi-pack-index write

# Check pack file integrity first
git verify-pack .git/objects/pack/*.pack

# Ensure no concurrent operations
rm -f .git/objects/pack/multi-pack-index.lock

# Check disk space
df -h .git/

Verification Errors:

# Detailed verification
git multi-pack-index verify --progress

# Check specific pack files
for pack in .git/objects/pack/*.pack; do
    echo "Checking $pack"
    git verify-pack "$pack" >/dev/null || echo "Failed: $pack"
done

# Rebuild if corrupted
git multi-pack-index expire
git multi-pack-index write

Performance Issues:

# Profile multi-pack-index operations
time git multi-pack-index write

# Check system resources
free -h
iostat -x 1 5

# Optimize system settings
echo 1 > /proc/sys/vm/drop_caches  # Clear cache
sysctl -w vm.dirty_ratio=10        # Reduce dirty memory

Bitmap Issues:

# Regenerate bitmap
git multi-pack-index expire
git repack -a -d --write-bitmap-index
git multi-pack-index --bitmap write

# Verify bitmap
git rev-list --use-bitmap-index --count HEAD

# Check bitmap file
ls -la .git/objects/pack/multi-pack-index-*.bitmap

Caution

Multi-pack-index operations can be resource-intensive on large repositories. Run during maintenance windows. Always backup before major repack operations.

Real-World Usage Examples:

Large Repository Optimization:

#!/bin/bash
# Optimize large enterprise repositories

enterprise_repo_optimization() {
    local repo_path="$1"

    cd "$repo_path"

    echo "Optimizing large repository: $(basename "$repo_path")"

    # Pre-optimization analysis
    echo "Before optimization:"
    du -sh .git/
    echo "Pack files: $(ls .git/objects/pack/*.pack | wc -l)"
    echo "Loose objects: $(find .git/objects -type f -name '[0-9a-f]*' | wc -l)"

    # Clean up loose objects
    git prune-packed
    git repack -f  # Force repack

    # Create multi-pack-index
    git multi-pack-index write --bitmap

    # Aggressive repack
    git multi-pack-index repack --max-pack-size=2g

    # Post-optimization analysis
    echo "After optimization:"
    du -sh .git/
    echo "Pack files: $(ls .git/objects/pack/*.pack | wc -l)"
    echo "Multi-pack-index: $(ls .git/objects/pack/multi-pack-index* | wc -l)"

    # Verify optimization
    git multi-pack-index verify

    # Update server info for clones
    git update-server-info

    echo "Enterprise optimization complete"
}

# Optimize critical repositories
enterprise_repo_optimization "/repos/critical-project"

Partial Clone Optimization:

# Optimize repositories for partial clones
partial_clone_optimization() {
    echo "Setting up repository for partial clone optimization"

    # Create multi-pack-index with bitmap
    git multi-pack-index --bitmap write

    # Generate pack bitmaps for reachability
    git repack -a -d --write-bitmap-index --write-bitmap-index-quiet

    # Configure for partial clone serving
    git config uploadpack.allowFilter true
    git config uploadpack.allowAnySHA1InWant true

    # Update server capabilities
    git update-server-info

    # Verify setup
    git multi-pack-index verify --bitmap

    echo "Partial clone optimization complete"
    echo "Clients can now use: git clone --filter=blob:none <url>"
}

partial_clone_optimization

Backup and Recovery Optimization:

# Optimize backup repositories
backup_optimization() {
    local backup_repo="$1"

    cd "$backup_repo"

    # Consolidate pack files
    git multi-pack-index write

    # Repack for optimal storage
    git multi-pack-index repack --batch-size=100

    # Verify backup integrity
    git multi-pack-index verify

    # Generate backup manifest
    cat > backup-manifest.txt << EOF
Backup Repository Manifest
Created: $(date)
Repository: $(basename "$backup_repo")
Multi-pack-index: $(ls .git/objects/pack/multi-pack-index* | wc -l)
Pack files: $(ls .git/objects/pack/*.pack | wc -l)
Total objects: $(git rev-list --all --count)
Repository size: $(du -sh .git/ | cut -f1)
EOF

    echo "Backup optimization complete"
    echo "Manifest saved: backup-manifest.txt"
}

# Optimize backup repositories
backup_optimization "/backups/project-archive"

Tip

Use multi-pack-index for repositories with many pack files. Enable bitmap support for better performance. Regular maintenance prevents index bloat.

What’s the difference between multi-pack-index and regular pack indexes?

Multi-pack-index provides unified indexing across multiple pack files; regular indexes are per-pack-file. MIDX enables efficient access to objects spread across many packs.

When should I create a multi-pack-index?

Create MIDX when repository has many pack files (>10) or frequent repacks. Useful for large repositories, shared object stores, and server optimization.

How does multi-pack-index affect clone performance?

MIDX with bitmaps enables faster clones by providing reachability information. Supports partial clones and reduces server load for large repositories.

Can multi-pack-index work with existing pack files?

Yes, MIDX works alongside existing pack files. Can index all packs in .git/objects/pack/ directory without modifying them.

What’s the impact of multi-pack-index on repository size?

MIDX adds small overhead (~1-2% of pack size) but enables better compression and reduces total repository size through optimized repacking.

How do I migrate from single-pack to multi-pack-index?

Run git multi-pack-index write to create MIDX for existing packs. No migration needed - MIDX works with existing pack structure.

Can multi-pack-index be used with Git LFS?

Yes, MIDX indexes Git objects including LFS pointers. LFS content management is separate from pack indexing.

What’s the relationship between multi-pack-index and git gc?

git gc can create MIDX during repacking. Use git multi-pack-index commands for manual control and advanced optimization.

How do I troubleshoot multi-pack-index corruption?

Use git multi-pack-index verify for integrity checking. Rebuild with git multi-pack-index expire && git multi-pack-index write if corrupted.

Can multi-pack-index work across multiple object directories?

Yes, with —object-dir option for alternates. Useful for shared object stores and distributed repository setups.

What’s the performance overhead of multi-pack-index?

Minimal read overhead, significant write overhead during creation. Benefits outweigh costs for repositories with many pack files.

How do I monitor multi-pack-index effectiveness?

Compare git rev-list —count performance with/without MIDX. Monitor pack file count and repository size changes after MIDX operations.

Can multi-pack-index be used in bare repositories?

Yes, operates on pack files regardless of working directory. Essential for server-side repository optimization.

What’s the difference between multi-pack-index and pack bitmaps?

MIDX indexes objects across packs; bitmaps provide reachability information. Both work together for optimal performance.

How do I handle multi-pack-index in CI/CD?

Create MIDX in CI for faster operations, but expire before artifact storage to reduce size. Use —no-bitmap for faster CI builds.

Can multi-pack-index work with incremental repacks?

Yes, supports incremental operations. Use git multi-pack-index repack for batch repacking based on MIDX analysis.

Applications of the git multi-pack-index command

1. Large Repository Optimization: Improve performance for repositories with many pack files through unified indexing
2. Server Performance Enhancement: Optimize Git servers for faster clones and reduced network usage
3. Storage Efficiency: Reduce repository size through intelligent pack file consolidation
4. Partial Clone Support: Enable efficient partial cloning with bitmap-optimized object access
5. Backup Optimization: Streamline backup processes for large repository archives
6. Enterprise Repository Management: Support large-scale repository operations in enterprise environments