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vfs: windows: fix slow opening of exe files by not truncating files when not necessary 

Before this change we truncated files in the backing store regardless
of whether we needed to or not.

After, we check to see if the file is the right size and don't
truncate if it is.

Apparently Windows Defender likes to check executables each time they
are modified, and truncating a file to its existing size is enough to
trigger the Windows Defender scan. This was causing a big slowdown for
operations which opened and closed the file a lot, such as looking at
properties on an executable.

See: https://forum.rclone.org/t/for-mount-sftp-why-right-click-on-exe-file-is-so-slow-until-it-freezes/33830
This commit is contained in:
Nick Craig-Wood 2022-11-10 18:00:32 +00:00
parent bb1fc5b86d
commit 652d3cdee4
1 changed files with 20 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
vfs/vfscache/item.go
View File

@ -272,11 +272,25 @@ func (item *Item) _truncate(size int64) (err error) {
}
}
fs.Debugf(item.name, "vfs cache: truncate to size=%d", size)
// Check to see what the current size is, and don't truncate
// if it is already the correct size.
//
// Apparently Windows Defender likes to check executables each
// time they are modified, and truncating a file to its
// existing size is enough to trigger the Windows Defender
// scan. This was causing a big slowdown for operations which
// opened and closed the file a lot, such as looking at
// properties on an executable.
fi, err := fd.Stat()
if err == nil && fi.Size() == size {
fs.Debugf(item.name, "vfs cache: truncate to size=%d (not needed as size correct)", size)
} else {
fs.Debugf(item.name, "vfs cache: truncate to size=%d", size)
err = fd.Truncate(size)
if err != nil {
return fmt.Errorf("vfs cache: truncate: %w", err)
err = fd.Truncate(size)
if err != nil {
return fmt.Errorf("vfs cache: truncate: %w", err)
}
}
item.info.Size = size
@ -460,7 +474,9 @@ func (item *Item) _createFile(osPath string) (err error) {
return errors.New("vfs cache item: internal error: didn't Close file")
}
item.modified = false
// t0 := time.Now()
fd, err := file.OpenFile(osPath, os.O_RDWR, 0600)
// fs.Debugf(item.name, "OpenFile took %v", time.Since(t0))
if err != nil {
return fmt.Errorf("vfs cache item: open failed: %w", err)
}