fsck: handle sparse files efficiently, fix xfstests generic/285

If we encounter a 128KB block of zeros, try to skip to the next
data section by calling File.SeekData().

This fixes xfstests generic/285, which creates a 17TB sparse file,
and runs fsck afterwards. Without this optimization, fsck would
take ages.

1 files changed, 11 insertions, 0 deletions

diff --git a/fsck.go b/fsck.go
index 37157cb..59bb2f0 100644
--- a/fsck.go
+++ b/fsck.go
@@ -1,6 +1,7 @@
 package main
 
 import (
+	"bytes"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -115,6 +116,7 @@ func (ck *fsckObj) file(path string) {
 		return
 	}
 	defer f.Release()
+	allZero := make([]byte, fuse.MAX_KERNEL_WRITE)
 	buf := make([]byte, fuse.MAX_KERNEL_WRITE)
 	var off int64
 	// Read() through the whole file and catch transparently mitigated corruptions
@@ -132,6 +134,15 @@ func (ck *fsckObj) file(path string) {
 			return
 		}
 		off += int64(result.Size())
+		// If we seem to be in the middle of a file hole, try to skip to the next
+		// data section.
+		if bytes.Equal(buf, allZero) {
+			f2 := f.(*fusefrontend.File)
+			nextOff, err := f2.SeekData(off)
+			if err == nil {
+				off = nextOff
+			}
+		}
 	}
 }