Samba共享传输大文件(ex:1G)失败的问题

时间:2022-07-02 17:18:13

1:问题描述

  1.1 基本信息

  遇见这样一个bug,路由器有USB share的功能,可将U盘内的文件通过samba和LAN端PC机中文件进行共享,测试发现小文件可正常共享,一旦文件大了(比如1G左右),window端便显示一直在计算文件大小,最后客户端(LAN pc)会因为服务器许久不回一个Subcommand:SET_FILE_INFO(0x0008)的报文而出现timeout,导致文件传输失败.

Samba共享传输大文件(ex:1G)失败的问题

过了一段时间后客户端发出timeout [RST] 报文(windows会弹出异常对话框取消文件传输)

Samba共享传输大文件(ex:1G)失败的问题

   重要的是对于NTFS的格式的U盘是正常的,只有FAT32格式的U盘有这个问题.

2:解决方法

  2.1 刚开始我觉得FAT32与NTFS的分区方式不同,可是了解到FAT32分区方式单个文件最大是4G,所以排除U盘本身问题,问题还是出在Samba服务器或者linux系统中(之所以怀疑linux系统是因为我在任何PC机上拷文件到此U盘都没问题)

  2.2 然后先debug samba源码,重点注意有关文件size和fat格式处理的相关代码,然后再源码中(\samba-3.0.24\source\smbd\vfs-wrap.c)看见了这个:

int vfswrap_ftruncate(vfs_handle_struct *handle, files_struct *fsp, int fd, SMB_OFF_T len)
{
    int result = -1;
    SMB_STRUCT_STAT st;
    char c = 0;
    SMB_OFF_T currpos;

    START_PROFILE(syscall_ftruncate);

    if (lp_strict_allocate(SNUM(fsp->conn))) {
        result = strict_allocate_ftruncate(handle, fsp, fd, len);
        END_PROFILE(syscall_ftruncate);
        return result;
    }

    /* we used to just check HAVE_FTRUNCATE_EXTEND and only use
       sys_ftruncate if the system supports it. Then I discovered that
       you can have some filesystems that support ftruncate
       expansion and some that don't! On Linux fat can't do
       ftruncate extend but ext2 can. */
    
    result = sys_ftruncate(fd, len);
    if (result == 0)
        goto done;
    
    /* According to W. R. Stevens advanced UNIX prog. Pure 4.3 BSD cannot
       extend a file with ftruncate. Provide alternate implementation
       for this */
    currpos = SMB_VFS_LSEEK(fsp, fd, 0, SEEK_CUR);
    if (currpos == -1) {
        goto done;
    }

    /* Do an fstat to see if the file is longer than the requested
       size in which case the ftruncate above should have
       succeeded or shorter, in which case seek to len - 1 and
       write 1 byte of zero */
    if (SMB_VFS_FSTAT(fsp, fd, &st) == -1) {
        goto done;
    }

#ifdef S_ISFIFO
    if (S_ISFIFO(st.st_mode)) {
        result = 0;
        goto done;
    }
#endif

    if (st.st_size == len) {
        result = 0;
        goto done;
    }

    if (st.st_size > len) {
        /* the sys_ftruncate should have worked */
        goto done;
    }

    if (SMB_VFS_LSEEK(fsp, fd, len-1, SEEK_SET) != len -1) goto done; 
    if (SMB_VFS_WRITE(fsp, fd, &c, 1)!=1)
        goto done;

    /* Seek to where we were */
    if (SMB_VFS_LSEEK(fsp, fd, currpos, SEEK_SET) != currpos)
        goto done;
    result = 0;

  done:

    END_PROFILE(syscall_ftruncate);
    return result;
}

   红色的两个函数是关键,在调试中发现,sys_ftruncate和SMB_VFS_WRITE在创建大一点的文件时会耗费很多的时间,这也是服务器迟迟不回复客户端SET_FILE_INFO请求的原因。所以将这两个函数屏蔽之后就正常了.

  也可以不屏蔽sys_ftruncate,可以将kernel的ftruncate函数实现修改一下:\linux-3.14\fs\fat\file.c 中函数fat_setattr

if (attr->ia_valid & ATTR_SIZE) {
        if (attr->ia_size > inode->i_size) {//if current size grenter than old size,extend it
        // if we allow this, fat ftruncate extend large size will cause samba wait a long long time.
        #if 1
            error = -EPERM;
            goto out;
        #else
            error = fat_cont_expand(inode, attr->ia_size);
            if (error || attr->ia_valid == ATTR_SIZE)
                goto out;
            attr->ia_valid &= ~ATTR_SIZE;
        #endif
        }
    }

  这样也可以解决问题.

 总结:

  不管是FAT32还是NTFS都是先接收数据然后写入到文件系统中,所以samba使用ftruncate函数扩展文件到文件系统中,我猜测可能是FAT32是把文件一次性使用fat_setattr函数写到文件系统中(大文件导致写入的时间过长),而NTFS是收到多少写入多少.这样导致FAT32不行而NTFS可行.