What is /proc/sys/vm/drop_caches?

🔍 What is /proc/sys/vm/drop_caches?

This is a virtual kernel interface that lets you manually free up memory used by:

• The page cache (file data)

• Dentries (directory entries)

• Inodes (metadata for files)

It’s useful when:

• You want to test performance without file cache interference

• You need to free up RAM quickly in a controlled environment

• You're benchmarking or tuning systems and want a clean cache state

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⚠️ Why cat drop_caches gives "Permission denied"

root@iamsonukushwaha:/proc/sys/vm# cat drop_caches
cat: drop_caches: Permission denied

Even as root, you can't read this file.

Here's why:

--w-------. 1 root root 0 Oct 6 15:48 drop_caches

• It’s write-only.

• There’s no read (r) permission — by design.

• The kernel doesn't expose a value here; it only accepts commands.

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✅ How to properly clear caches

Before you drop caches, it’s important to sync your file system to avoid data loss:

sync

Then, choose the appropriate option:

# 1: Clear page cache
echo 1 > /proc/sys/vm/drop_caches

# 2: Clear dentries and inodes
echo 2 > /proc/sys/vm/drop_caches

# 3: Clear page cache, dentries, and inodes
echo 3 > /proc/sys/vm/drop_caches

✅ Recommended safe combo:

sync && echo 3 > /proc/sys/vm/drop_caches