worktree: copy-on-write creation and shared-branch worktrees#2317
worktree: copy-on-write creation and shared-branch worktrees#2317nevion wants to merge 2 commits into
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Welcome to GitGitGadgetHi @nevion, and welcome to GitGitGadget, the GitHub App to send patch series to the Git mailing list from GitHub Pull Requests. Please make sure that either:
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this feature is used/reimplemented within grok build https://x.com/theskory/status/2059729539287167068 (I have nothing to do with that) but the best place for this to be done is inside git so all tools can benefit. |
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Benchmark — Linux-kernel fork (93k tracked files, ~33 GB working tree incl. build artifacts, btrfs):
Left: real disk written vs. number of worktrees (normal grows ~1.3 GB each; |
Creating many worktrees from the same base -- for example to run a
fleet of automated agents in parallel -- is expensive today: every
"git worktree add" materializes the entire working tree by writing
each tracked file out from the object store. The objects are shared
via the common directory, but the working tree is not: N worktrees
mean N full checkouts on disk and N times the file I/O.
Add a "--reflink" option that, on copy-on-write filesystems, populates
the new worktree by reflinking the current worktree's files and index
instead. The subsequent "git reset --hard" then only rewrites the
paths that actually differ between the current worktree and
<commit-ish>; everything else (including untracked files such as build
outputs) keeps sharing storage with the source until modified. Because
the cloned index still carries the source files' stat data, it is
refreshed against the reflinked files first so that reset recognizes
the unchanged paths as up to date and leaves them sharing extents
rather than rewriting them.
The clones are made by a new reflink_file() helper in copy.c, which
uses the FICLONE ioctl on Linux and clonefile() on macOS and reports
an error otherwise so callers fall back to a normal copy. Support is
probed up front; when unavailable -- including on filesystems without
copy-on-write and on platforms such as Windows that lack a reflink
primitive -- "--reflink" transparently falls back to an ordinary
checkout, so the worst case is no slower than today rather than a
byte-for-byte copy of the source tree. The directory walk skips the
new worktree itself when it lives inside the source one, and preserves
symlinks and modes.
The behavior can be made the default with the worktree.reflink
configuration ("true", "false" or "auto", the last suppressing the
unsupported-filesystem warning), and turned off per-invocation with
--no-reflink. A configured default degrades quietly in modes that
cannot reflink (--orphan, --no-checkout) instead of erroring, so
enabling it never breaks those commands. The checkout step continues
to honor checkout.workers, so parallel checkout composes with
--reflink for the paths that do need rewriting.
Signed-off-by: Jason Newton <nevion@gmail.com>
When spinning up several worktrees on the same checkout for parallel work (for example a fleet of agents working from one branch), git's refusal to check out a branch that is already checked out elsewhere is just in the way. The restriction exists to stop two worktrees from moving the same branch underneath each other, but plain parallel checkouts do not need that protection. Drop the restriction: "git worktree add <branch>" now checks out a branch even if it is in use by another worktree. The genuinely dangerous case is kept -- a branch that another worktree is in the middle of rebasing or bisecting is still refused, because a second checkout could corrupt that operation. die_if_branch_busy() performs that narrower check in place of the old die_if_checked_out(). The separate guard against force-updating (e.g. with -B) a branch in use elsewhere is left untouched. Signed-off-by: Jason Newton <nevion@gmail.com>
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The non-Linux CI failures here are unrelated to this change (which only touches
Every job that actually exercises this change (all |
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/allow |
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Yes, sadly Git's CI is not always in a pristine shape :-( |
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User nevion is now allowed to use GitGitGadget. |
When many worktrees share one repository -- e .g. a fleet of agents each
needing an isolated checkout -- "git worktree add" is costly at scale.
Objects are shared via the common dir, but the working tree is not: each
add rewrites every tracked file, so N worktrees cost N full checkouts of
disk and I/O. And a branch can only be checked out in one worktree.
Patch 1 adds "git worktree add --reflink": on a copy-on-write filesystem
it populates the new worktree by reflinking the current worktree's files
and index, then "git reset --hard" rewrites only the paths that differ
from . A reflink_file() helper in copy.c uses FICLONE (Linux)
and clonefile() (macOS); elsewhere (other filesystems, Windows) it is
probed up front and falls back to a normal checkout. Defaulting is via
the worktree.reflink config (true/false/auto); --no-reflink overrides.
Patch 2 lets a branch be checked out in several worktrees, for parallel
work on one checkout. A branch mid-rebase or mid-bisect elsewhere is
still refused.
Benchmark (Linux-kernel fork, 93k files, ~33 GB tree incl. build output,
btrfs): a normal add allocates ~0.9 GB of real disk per worktree (~5.3 GB
for four, linear); --reflink allocates ~0 at any count and also carries
the untracked build tree. ("Real disk" = btrfs exclusive bytes.)
Note: patch 2 changes a default (same-branch checkout now allowed); two
t2400 assertions were updated accordingly.