Understanding Hole Punching for Leaner Firecracker Memory Snapshots

Memory snapshots are indispensable to millisecond-scale boot in MicroVM environments such as Firecracker. In large-scale infrastructure that has to create and retain snapshots for a great many AI agent VMs, however, using them creates a dilemma of its own: storage capacity runs short and costs rise. Against that background, this article examines the storage optimization principle that identifies the dead weight in snapshot data and cuts disk usage by more than 90% using the hole punching facility the OS provides.

The dilemma: the gap between total RAM size and the real working set

Recording a VM's state to a snapshot as it stands normally produces a file on disk the same size as the total guest RAM allocated to it. An AI agent VM running with 512 MB of RAM, for example, dumps a 512 MB memory.bin file straight to disk. The way data is actually distributed inside that memory is a very different matter. The baseline memory needed for the guest OS kernel to finish booting, bring up the virtual network stack, and for the in-VM agent to open its communication sockets and settle into a waiting state — the effective working set — usually amounts to no more than 43–65 MB. The rest of that vast memory space is filled with pure invalid data (zeros) that has not been touched once since start-up. Recording the whole of memory exactly as it is amounts to serious storage waste: at 512 MB of RAM roughly 92% of the disk space, and at 1024 MB roughly 94%, is thrown away storing meaningless zeros.

The remedy: creating a sparse file with hole punching

The systems programming technique that addresses this waste at its root is hole punching. It works by manipulating file system metadata to break the mapping between the logical offsets that invalid data occupied and the physical disk blocks behind them, returning those blocks to the system as free space. In concrete terms, the optimization pipeline works as follows.

• Locating the data regions: the SEEK_DATA and SEEK_HOLE system calls are used to walk the real data regions and the empty regions inside the snapshot file efficiently.
• Identifying zero blocks: during that walk, 4 KiB blocks consisting entirely of zeros are detected and consecutive runs are merged into a single chunk.
• Unmapping the blocks: fallocate(PUNCH_HOLE | KEEP_SIZE) is called on each detected zero region. This is a mechanism that preserves the overall logical size of the file while deallocating and reclaiming only the physical disk blocks that were mapped to that region.

Dramatic storage savings

What matters about hole punching is that the logical size of the file is preserved exactly as in the original while the block size actually allocated on disk shrinks dramatically, turning the file into a sparse file. The measurements demonstrate the force of this file system optimization plainly.

• At 512 MB of RAM: the snapshot file keeps its logical size of 512 MB, but the physical size actually allocated on disk drops sharply to around 43 MB, close to the effective working set.
• That represents a disk footprint reduction of fully 92%.
• At 1024 MB and 2048 MB, the disk space actually allocated likewise does not grow in proportion to RAM size but holds the line at 43–65 MB. The larger the memory allocated to a VM, in other words, the greater the storage saving becomes.

Restore performance and data integrity

The most important question is whether restoring a VM from a snapshot file whose physical blocks have been deallocated causes any slowdown or data corruption. Measurement shows no adverse effect whatsoever on restore speed or data integrity. During a VM restore, the Firecracker process maps the snapshot file into its memory space and lazy-loads from it. The file system intercepts requests that touch a deallocated region — a hole — and, rather than issuing disk I/O, immediately returns a zero-filled page from memory. From the application's point of view the behavior is therefore exactly identical to reading the original dump file, and the system resumes intact with no additional computational overhead for the restore.