ceremonyclient/pebble/vfs/default_linux.go

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2024-01-03 07:31:42 +00:00
// Copyright 2023 The LevelDB-Go and Pebble Authors. All rights reserved. Use
// of this source code is governed by a BSD-style license that can be found in
// the LICENSE file.
//go:build linux
// +build linux
package vfs
import (
"os"
"syscall"
"github.com/cockroachdb/errors"
"golang.org/x/sys/unix"
)
func wrapOSFileImpl(f *os.File) File {
lf := &linuxFile{File: f, fd: f.Fd()}
if lf.fd != InvalidFd {
lf.useSyncRange = isSyncRangeSupported(lf.fd)
}
return lf
}
func (defaultFS) OpenDir(name string) (File, error) {
f, err := os.OpenFile(name, syscall.O_CLOEXEC, 0)
if err != nil {
return nil, errors.WithStack(err)
}
return &linuxDir{f}, nil
}
// Assert that linuxFile and linuxDir implement vfs.File.
var (
_ File = (*linuxDir)(nil)
_ File = (*linuxFile)(nil)
)
type linuxDir struct {
*os.File
}
func (d *linuxDir) Prefetch(offset int64, length int64) error { return nil }
func (d *linuxDir) Preallocate(offset, length int64) error { return nil }
func (d *linuxDir) SyncData() error { return d.Sync() }
func (d *linuxDir) SyncTo(offset int64) (fullSync bool, err error) { return false, nil }
type linuxFile struct {
*os.File
fd uintptr
useSyncRange bool
}
func (f *linuxFile) Prefetch(offset int64, length int64) error {
_, _, err := unix.Syscall(unix.SYS_READAHEAD, uintptr(f.fd), uintptr(offset), uintptr(length))
return err
}
func (f *linuxFile) Preallocate(offset, length int64) error {
return unix.Fallocate(int(f.fd), unix.FALLOC_FL_KEEP_SIZE, offset, length)
}
func (f *linuxFile) SyncData() error {
return unix.Fdatasync(int(f.fd))
}
func (f *linuxFile) SyncTo(offset int64) (fullSync bool, err error) {
if !f.useSyncRange {
// Use fdatasync, which does provide persistence guarantees but won't
// update all file metadata. From the `fdatasync` man page:
//
// fdatasync() is similar to fsync(), but does not flush modified
// metadata unless that metadata is needed in order to allow a
// subsequent data retrieval to be correctly handled. For example,
// changes to st_atime or st_mtime (respectively, time of last access
// and time of last modification; see stat(2)) do not require flushing
// because they are not necessary for a subsequent data read to be
// handled correctly. On the other hand, a change to the file size
// (st_size, as made by say ftruncate(2)), would require a metadata
// flush.
if err = unix.Fdatasync(int(f.fd)); err != nil {
return false, err
}
return true, nil
}
const (
waitBefore = 0x1
write = 0x2
// waitAfter = 0x4
)
// By specifying write|waitBefore for the flags, we're instructing
// SyncFileRange to a) wait for any outstanding data being written to finish,
// and b) to queue any other dirty data blocks in the range [0,offset] for
// writing. The actual writing of this data will occur asynchronously. The
// use of `waitBefore` is to limit how much dirty data is allowed to
// accumulate. Linux sometimes behaves poorly when a large amount of dirty
// data accumulates, impacting other I/O operations.
return false, unix.SyncFileRange(int(f.fd), 0, offset, write|waitBefore)
}
type syncFileRange func(fd int, off int64, n int64, flags int) (err error)
// sync_file_range depends on both the filesystem, and the broader kernel
// support. In particular, Windows Subsystem for Linux does not support
// sync_file_range, even when used with ext{2,3,4}. syncRangeSmokeTest performs
// a test of of sync_file_range, returning false on ENOSYS, and true otherwise.
func syncRangeSmokeTest(fd uintptr, syncFn syncFileRange) bool {
err := syncFn(int(fd), 0 /* offset */, 0 /* nbytes */, 0 /* flags */)
return err != unix.ENOSYS
}
func isSyncRangeSupported(fd uintptr) bool {
var stat unix.Statfs_t
if err := unix.Fstatfs(int(fd), &stat); err != nil {
return false
}
// Allowlist which filesystems we allow using sync_file_range with as some
// filesystems treat that syscall as a noop (notably ZFS). A allowlist is
// used instead of a denylist in order to have a more graceful failure mode
// in case a filesystem we haven't tested is encountered. Currently only
// ext2/3/4 are known to work properly.
const extMagic = 0xef53
switch stat.Type {
case extMagic:
return syncRangeSmokeTest(fd, unix.SyncFileRange)
}
return false
}