为什么.NET的File.Open使用UNC路径进行过多的SMB调用？

Question

我有一段代码需要使用UNC路径从NAS服务器打开和读取大量小文本文件.此代码是最初用C++编写但现在转换为C#的模块的一部分.C#版本明显变慢了.我确定打开文件的调用几乎占据了所有的性能差异.使用WireShark我发现这是因为System.IO.File.Open调用比类似的C++代码产生更多的SMB网络请求.

C++代码进行此调用:

FILE *f = _wfsopen(fileName, L"r", _SH_DENYWR);

这导致以下SMB请求序列:

NT Create AndX Request, FID: 0x0004, Path: \\a\\i\\a\\q\\~141106162638847.nmd
NT Create AndX Response, FID: 0x0004
Trans2 Request, QUERY_FILE_INFO, FID: 0x0004, Query File Basic Info
Trans2 Response, FID: 0x0004, QUERY_FILE_INFO
Read AndX Request, FID: 0x0004, 1327 bytes at offset 0
Read AndX Response, FID: 0x0004, 1327 bytes
Close Request, FID: 0x0004
Close Response, FID: 0x0004
NT Create AndX Request, FID: 0x0005, Path: \\a\\i\\a\\q\\~141106162638847.nmd
NT Create AndX Response, FID: 0x0005

C#代码进行此调用:

FileStream f = File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.Read);

这导致以下SMB请求序列:

Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \\a\\i\\a\\q\\~141106162638847.nmd
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \\a\\i\\a\\q\\~141106162638847.nmd
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \\a\\i\\a\\q\\~141106162638847.nmd
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: 
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: 
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \\a
Trans2 Response, FIND_FIRST2, Files: a
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \\a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \\a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \\a\\i
Trans2 Response, FIND_FIRST2, Files: i
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \\a\\i
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \\a\\i
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \\a\\i\\a
Trans2 Response, FIND_FIRST2, Files: a
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \\a\\i\\a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \\a\\i\\a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \\a\\i\\a\\q
Trans2 Response, FIND_FIRST2, Files: q
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \\a\\i\\a\\q\\~141106162638847.nmd
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \\a\\i\\a\\q\\~141106162638847.nmd
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \\a\\i\\a\\q\\~141106162638847.nmd
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: 
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: 
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \\a
Trans2 Response, FIND_FIRST2, Files: a
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \\a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \\a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \\a\\i
Trans2 Response, FIND_FIRST2, Files: i
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \\a\\i
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \\a\\i
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \\a\\i\\a
Trans2 Response, FIND_FIRST2, Files: a
Trans2 Request, QUERY_PATH_INFO, Query File Basic Info, Path: \\a\\i\\a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, QUERY_PATH_INFO, Query File Standard Info, Path: \\a\\i\\a
Trans2 Response, QUERY_PATH_INFO
Trans2 Request, FIND_FIRST2, Pattern: \\a\\i\\a\\q
Trans2 Response, FIND_FIRST2, Files: q
Close Request, FID: 0x000f
Close Response
NT Create AndX Request, FID: 0x0018, Path: \\a\\i\\a\\q\\~141106162638847.nmd
NT Create AndX Response, FID: 0x0018
Trans2 Request, QUERY_FILE_INFO, FID: 0x0018, Query File Basic Info
Trans2 Response, FID: 0x0018, QUERY_FILE_INFO
Read AndX Request, FID: 0x0018, 1327 bytes at offset 0
Read AndX Response, FID: 0x0018, 1327 bytes
Close Request, FID: 0x0018
Close Response, FID: 0x0018
NT Create AndX Request, FID: 0x0019, Path: \\a\\i\\a\\q\\~141106162638847.nmd
NT Create AndX Response, FID: 0x0019

为什么System.IO.File.Open会发出所有这些额外的SMB请求？有没有办法更改此代码以避免所有这些额外的请求？

Answer 1

简而言之,File.Open调用new FileStream()并new FileStream()执行大量调用:

1. NormalisePath.

   String filePath = Path.NormalizePath(path, true, maxPath); // fullCheck: true
   

导致这段代码:

1.a:获取完整路径:

    if (fullCheck) { ... 
        result = newBuffer.GetFullPathName();

GetFullPathName()调用Win32Native.GetFullPathName一次或两次(取决于生成的路径的lentgh).

1.B. 试图扩大短途径.您的路径包含~char,因此它看起来像路径扩展的候选者:

    if (mightBeShortFileName) {
        bool r = newBuffer.TryExpandShortFileName();

结果,Win32Native.GetLongPathName()被调用.

2. FileIoPermission.Demand()(仅适用于非信任):

   // All demands in full trust domains are no-ops, so skip 
   if (!CodeAccessSecurityEngine.QuickCheckForAllDemands()) {
       ...
       new FileIOPermission(secAccess, control, new String[] { filePath }, false, false).Demand();
   

3. 打开fileStream(软盘反击;)):

    // Don't pop up a dialog for reading from an emtpy floppy drive
   int oldMode = Win32Native.SetErrorMode(Win32Native.SEM_FAILCRITICALERRORS);
   try {
       ...
       _handle = Win32Native.SafeCreateFile(tempPath, fAccess, share, secAttrs, mode, flagsAndAttributes, IntPtr.Zero);
   

4. Win32Native.GetFileType()

并非所有这些都会导致smb请求,但有些人会这样做.我试图通过逐步调试(这里是启用.net源调试的手册)并在每个步骤后检查日志来重现繁琐的请求.Resuts与您的第一个列表更相似.如果你真的有兴趣找到真正的问题,你必须自己做.

UPD请注意,我已检查当前(.net 4.5.2)行为.它自2.0以来被多次更改(例如FileIOPermission.Demand()最初也被称为完全受信任的代码),所以它取决于:)

Answer 2

我真的没有具体的答案为什么.NET实现如此繁琐,但这种行为将由于System.IO.FileStream所有File.Open(fileName, FileMode.Open, FileAccess.Read, FileShare.Read);正在做的是将参数传递给FileStream构造函数.

public static FileStream Open(string path, FileMode mode, FileAccess access, FileShare share)
{
    return new FileStream(path, mode, access, share);
}

更改FileStream的行为意味着您基本上必须重新实现FileStream类,这将需要很多努力.

另一个更简单的替代方法是创建一个调用您提供的C++代码的本机包装器.然后从C#代码中调用本机包装器.