RightSignal · Sunday, November 8

Concept of the day1 / 3

Copy-on-write

The concept

Why duplicating a huge file on a Mac can finish instantly

Copy-on-write means a copy does not duplicate any data at first. The original and the copy point at the same blocks on disk or in memory, and a block is duplicated only when one side tries to change it.

Duplicate a 10 GB video in Finder on an Apple APFS drive and it appears at once. The new file is just a fresh name pointing at the same blocks. Only when you edit part of one file does the system write new blocks for the changed part.

Concept of the day2 / 3

Copy-on-write

Real-life use case

Linux's fork() makes a new process without copying its memory

When a Linux program calls fork() to start a child process, the kernel does not copy the parent's memory. Both processes share the same memory pages (4 KB chunks), and the kernel marks them read-only.

File systems like ZFS and Btrfs use the same trick for snapshots: a snapshot of a whole disk takes moments, because it only keeps references to blocks that already exist.

Concept of the day3 / 3

Copy-on-write

How it helps

Shared blocks get copied only at the moment someone writes

Reading a shared block is free. A write to a read-only page makes the hardware trap to the kernel (a page fault); the kernel copies just that page, hands the writer its private copy, and lets the write go ahead.

So the cost follows what changes, not the original's size. A child process that reads a lot but writes little costs almost nothing, and since old blocks are never overwritten, a snapshot can keep pointing at them.

What it’s forEstimates how much space an original and its copy take under copy-on-write.

storage = N + c

N: blocks in the original, all still shared after the copy
c: blocks changed since the copy, each now stored twice

What it givesTotal blocks used, which grows only with the edits, not with the copy itself.

Original file: N = 1,000 blocks
Copy it, then edit 5 blocks: c = 5
storage = 1,000 + 5 = 1,005 blocks, not 2,000

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Topics in News

Why duplicating a huge file on a Mac can finish instantly

Linux's fork() makes a new process without copying its memory

Shared blocks get copied only at the moment someone writes

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