Why Does Vim Stall on 10 GB Files? Inside Its IO and Save Mechanics

Background

The author opened a 10 GB file in Vim, edited a single line, and noticed that saving with :w took minutes, prompting a deep dive into Vim's internal I/O behavior.

Vim as a Terminal Editor

Vim is bundled with virtually every Linux distribution and remains essential for remote terminal editing where graphical editors are unavailable.

Process Initialization

Running vim test.txt creates a process that starts at main, performs early machine‑specific initialization ( mch_early_init()), sets global parameters ( common_init(&params)), loads the command table, sources .vimrc and other startup scripts, initializes the terminal ( termcapinit), and finally creates windows and buffers.

Opening a File – readfile

The core function readfile (about 2,500 lines) reads the entire file into Vim's virtual memory abstraction called memfile. It creates a memline buffer, may generate a swap file ( .swp) for recovery, and decodes the data for display. This full read explains why opening a 10 GB file can take over a minute.

Memory Layout – memfile and Blocks

Vim implements its own virtual storage: the file is represented as a tree of 4 KB blocks. Block 0 stores metadata (Vim version, file path, encoding). Pointer blocks form the internal tree, and data blocks hold the actual user content. The swap file mirrors this structure; the first 4 KB is a header, the rest are blocks.

Saving with :w

When the user types :w, Vim invokes the callback ex_write, which eventually calls buf_write. The sequence is:

Create a backup file test.txt~ by copying the original.

Truncate the original file to zero ( ftruncate).

Copy the current contents of the memfile (memory + .test.txt.swp) back into test.txt.

Delete the backup file.

Both the backup copy and the final rewrite involve a full read/write of the file, so saving a 10 GB file also takes on the order of minutes.

Swap File vs Backup File

The .test.txt.swp file is an always‑open swap file used by the memfile for crash recovery; it is not a user‑visible backup. The test.txt~ file is the true backup created only during :w and removed after a successful write.

Space Consumption

During a write operation Vim may need more than 30 GB of disk space because the original file, the backup, and the swap file can coexist.

Takeaways

Vim relies on plain read, write, and lseek system calls; there is no hidden magic.

Opening a large file is slow because readfile reads the entire file into memory.

Saving is slow because Vim copies the whole file twice (backup + rewrite).

The internal memfile abstraction provides a virtual block‑based view of the file, with block 0 metadata, pointer blocks, and data blocks.

Vim is not suited for editing multi‑gigabyte files.