bazelbuild/bazel

Bazel digest calculation of big sparse files is slow

Open

#25,265 opened on Feb 12, 2025

View on GitHub
 (2 comments) (0 reactions) (0 assignees)Java (25,384 stars) (4,465 forks)batch import
P3help wantedteam-Performancetype: feature request

Description

Description of the bug:

We use Bazel to build partition and disk images. The individual partitions are built across multiple build steps and assembled into a disk image in the end.

The images are stored as sparse files in the filesystem and their disk size is 1-10% of their logical size. For example: 100GB size but only 2GB on disk. When Bazel computes the digest of the images, it cannot take advantage of the sparse nature of the file (e.g. by using SEEK_HOLE and SEEK_DATA) and reads the giant holes which is very slow.

While DigestFunction in remote_execution.proto does have a few digest functions that could be easily implemented to work optimally with sparse files, these are not available in the hashFunctionRegistry. Only BLAKE3, SHA1 and SHA256 are available.

Which category does this issue belong to?

Performance

What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

genrule(
    name = "big_sparse_file",
    outs = ["big_sparse_file.dat"],
    cmd = "truncate $@ -s 10G",
)

INFO: Elapsed time: 23.773s, Critical Path: 14.13s

Which operating system are you running Bazel on?

Ubuntu 20.04

What is the output of bazel info release?

release 7.4.1

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

No response

If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

No response

Have you found anything relevant by searching the web?

No response

Any other information, logs, or outputs that you want to share?

No response

Contributor guide

Tech stack
java
Domain
backendbuild systemperformance
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Familiarity with Bazel's Virtual File SystemJava developmentUnderstanding of sparse files
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
Investigate the implementation of DigestHashFunction in bazel's source code (src/main/java/com/google/devtools/build/lib/vfs/DigestHashFunction.java). The issue suggests that adding a digest function that supports sparse files efficiently using SEEK HOLE/SEEK DATA would solve the problem. Also review the remote execution.proto for digest function definitions. There are 2 comments on the issue that may provide further insight into the desired approach.