106 lines
3.1 KiB
Go
106 lines
3.1 KiB
Go
// Copyright 2019 The LevelDB-Go and Pebble Authors. All rights reserved. Use
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// of this source code is governed by a BSD-style license that can be found in
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// the LICENSE file.
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//go:build linux && !arm
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// +build linux,!arm
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package vfs
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import "syscall"
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type syncFileRange func(fd int, off int64, n int64, flags int) (err error)
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// sync_file_range depends on both the filesystem, and the broader kernel
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// support. In particular, Windows Subsystem for Linux does not support
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// sync_file_range, even when used with ext{2,3,4}. syncRangeSmokeTest performs
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// a test of of sync_file_range, returning false on ENOSYS, and true otherwise.
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func syncRangeSmokeTest(fd uintptr, fn syncFileRange) bool {
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err := fn(int(fd), 0 /* offset */, 0 /* nbytes */, 0 /* flags */)
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return err != syscall.ENOSYS
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}
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func isSyncRangeSupported(fd uintptr) bool {
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var stat syscall.Statfs_t
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if err := syscall.Fstatfs(int(fd), &stat); err != nil {
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return false
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}
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// Allowlist which filesystems we allow using sync_file_range with as some
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// filesystems treat that syscall as a noop (notably ZFS). A allowlist is
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// used instead of a denylist in order to have a more graceful failure mode
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// in case a filesystem we haven't tested is encountered. Currently only
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// ext2/3/4 are known to work properly.
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const extMagic = 0xef53
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switch stat.Type {
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case extMagic:
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return syncRangeSmokeTest(fd, syscall.SyncFileRange)
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}
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return false
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}
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func (f *syncingFile) init() {
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if f.fd == 0 {
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return
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}
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f.timeDiskOp(func() {
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f.useSyncRange = isSyncRangeSupported(f.fd)
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})
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if f.useSyncRange {
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f.syncTo = f.syncToRange
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} else {
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f.syncTo = f.syncToFdatasync
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}
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f.syncData = f.syncFdatasync
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}
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func (f *syncingFile) syncFdatasync() error {
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if f.fd == 0 {
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return f.File.Sync()
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}
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var err error
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f.timeDiskOp(func() {
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err = syscall.Fdatasync(int(f.fd))
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})
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return err
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}
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func (f *syncingFile) syncToFdatasync(_ int64) error {
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return f.Sync()
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}
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func (f *syncingFile) syncToRange(offset int64) error {
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const (
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waitBefore = 0x1
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write = 0x2
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// waitAfter = 0x4
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)
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// The flags for the sync_file_range system call. Unless the file has
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// noSyncOnClose explicitly set and it is being closed, the waitBefore
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// flag will be set which may block the call.
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flags := write
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if !f.noSyncOnClose || !f.closing {
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flags |= waitBefore
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}
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// Note that syncToRange is only called with an offset that is guaranteed to
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// be less than atomic.offset (i.e. the write offset). This implies the
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// syncingFile.Close will Sync the rest of the data, as well as the file's
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// metadata.
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f.ratchetSyncOffset(offset)
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// By specifying write|waitBefore for the flags, we're instructing
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// SyncFileRange to a) wait for any outstanding data being written to finish,
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// and b) to queue any other dirty data blocks in the range [0,offset] for
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// writing. The actual writing of this data will occur asynchronously. The
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// use of `waitBefore` is to limit how much dirty data is allowed to
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// accumulate. Linux sometimes behaves poorly when a large amount of dirty
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// data accumulates, impacting other I/O operations.
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var err error
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f.timeDiskOp(func() {
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err = syscall.SyncFileRange(int(f.fd), 0, offset, flags)
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})
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return err
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}
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