During a recent disk usage analysis on two identical Dell PE2850 servers running RHEL5, I noticed something peculiar. The du -sh /opt/foobar command took 5 minutes to complete on Server A (with ~25GB data), while executing instantly on Server B with identical data. This performance gap raised several questions about disk analysis efficiency.

After thorough investigation, several factors emerged as possible culprits for the slow du performance:

• Filesystem differences: Server A might be using ext3 with slow directory indexing
• Disk health issues: Bad sectors forcing repeated reads
• Mount options: Different noatime/nodiratime settings
• Background processes: Antivirus or backup software scanning files
• Directory structure: Millions of small files vs. fewer large files

To pinpoint the exact cause, run these commands on both servers:

# Check filesystem type and mount options
mount | grep /opt

# Check disk I/O performance
hdparm -tT /dev/sdX

# Monitor disk activity during du execution
iostat -x 1

# Alternative counting method
find /opt/foobar -type f -printf '%s\n' | awk '{total+=$1} END {print total}'

For faster disk usage analysis, consider these approaches:

# 1. Use parallel processing (GNU parallel required)
find /opt/foobar -type d | parallel du -s | awk '{total+=$1} END {print total}'

# 2. Try ncdu (NCurses Disk Usage)
yum install ncdu
ncdu /opt/foobar

# 3. Exclude certain directories
du -sh /opt/foobar --exclude='*/cache/*'

# 4. Filesystem-specific optimizations
tune2fs -O dir_index /dev/sdX  # For ext3/ext4
xfs_repair /dev/sdX           # For XFS

Here's a benchmark of different methods on a test directory with 500,000 files:

Remember that results vary based on filesystem type, disk speed, and directory structure. The parallel processing method shows particularly good scaling for systems with multiple CPU cores.

For extreme cases with millions of files, consider these advanced techniques:

# 1. Use inode-based counting (works even when directory is unreadable)
find /opt/foobar -printf '%i\n' | wc -l

# 2. Compile a custom C program for maximum speed
#include 
#include 
#include 

long long du(const char *path) {
  struct stat st;
  if (lstat(path, &st) == -1) return 0;
  if (!S_ISDIR(st.st_mode)) return st.st_blocks;
  
  long long total = st.st_blocks;
  DIR *dir = opendir(path);
  if (!dir) return total;
  
  struct dirent *entry;
  while ((entry = readdir(dir)) != NULL) {
    if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0) continue;
    char subpath[PATH_MAX];
    snprintf(subpath, PATH_MAX, "%s/%s", path, entry->d_name);
    total += du(subpath);
  }
  closedir(dir);
  return total;
}

int main(int argc, char **argv) {
  printf("%lld\t%s\n", du(argv[1]) / 2, argv[1]);
  return 0;
}

When running du -sh /opt/foobar on identical RHEL5 servers (Dell PE2850s), I noticed significant performance differences. While server B returns results instantly for 25GB directories, server A takes about 5 minutes to complete the same operation.

Several factors could cause this discrepancy:

# Check for filesystem differences
$ mount | grep /opt
$ df -Th /opt

# Verify disk health
$ smartctl -a /dev/sdX
$ iostat -x 1 5

The filesystem type significantly impacts du performance:

# For ext filesystems, try:
$ tune2fs -l /dev/sdX | grep features
$ debugfs -R "stats" /dev/sdX | grep -i fragmentation

# For XFS:
$ xfs_db -c frag /dev/sdX

When du is slow, consider these alternatives:

# Use ncdu for interactive analysis
$ ncdu /opt/foobar

# Try the faster but less accurate method
$ ls -lR /opt/foobar | awk '{sum += $5} END {print sum}'

# Parallel processing approach
$ find /opt/foobar -type f -print0 | xargs -0 -n1 -P$(nproc) du -s | awk '{sum+=$1} END {print sum}'

Compare these key parameters between servers:

$ sysctl vm.dirty_ratio vm.dirty_background_ratio
$ grep -i 'swap' /proc/meminfo
$ cat /proc/sys/fs/file-nr
$ ulimit -a

These tweaks often help:

# Disable atime updates
$ mount -o remount,noatime /opt

# Increase inode cache
$ sysctl -w vm.vfs_cache_pressure=50

# Adjust readahead for HDDs
$ blockdev --setra 4096 /dev/sdX

# Clear caches (careful with production systems)
$ sync; echo 3 > /proc/sys/vm/drop_caches

For persistent issues, use these advanced tools:

# Trace system calls
$ strace -c du -sh /opt/foobar

# IO profiling
$ iotop -oPa

# Detailed filesystem profiling
$ fsmark -d /opt/foobar -s 100 -n 1000