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有时候,我们希望我们的数据不通过page cache的缓冲直接落盘。go语言里,用参数DIRECT打开文件可以实现这一点要求。 但这样做有一个硬性的要求,就是在读写的时候,对应的数据在内存中的地址一定要满足512对齐,即首地址的2进制形式中后面至少要有9个0结尾,且数据长度为512字节的整数倍,否则读写会失败。 我们用go语言中的切片slice来验证这件事。 首先我们建立一个go语言的切片并随便赋一些值: buf := make([]byte, 8192) for i := 0; i < 20; i++ { buf[i] = byte(i) } 我们首先尝试一下正常的读写文件过程,先不使用DIRECT参数绕开page cache。 func writeWithoutAlignmentWithoutDIRECT(buf []byte) { // open file file, err := os.OpenFile("/dev/sdb", os.O_WRONLY|os.O_CREATE, 0666) if err != nil { fmt.Printf("An error occurred with file opening or creation\n") return } defer file.Close() // write file fmt.Println("buffer ", unsafe.Pointer(&buf)) fmt.Println("buffer[0] ", unsafe.Pointer(&buf[0])) buf2 := buf[4:516] fmt.Println("write with buffer ", unsafe.Pointer(&buf2[0])) _, err = file.WriteAt(buf2, 512) if err != nil { fmt.Println("write error ", err) } else { fmt.Println("write succeed") } } 这段代码的运行结果如下: buffer 0xc42000a2a0 buffer[0] 0xc42005a000 write with buffer 0xc42005a004 write succeed 可以看出,这个切片的地址是0xc42000a2a0,这个切片内数据的首地址是0xc42005a000,这是一个与c语言不同的地方,c语言的数据首地址即为其中数据的首地址,而go语言中,切片的地址和切片内数据首地址是不同的。 我们要写入的数据的首地址是从切片的第5个元素开始,其首地址为0xc42005a004,虽然并没有512对齐,但是由于我们没有尝试绕过page cache,所以根据WriteAt函数的返回值err可以看到,我们的写入操作是成功的。 下面我们尝试一下使用DIRECT参数打开文件,绕过page cache进行写数据操作,其他参数不变。 func writeWithoutAlignmentWithDIRECT(buf []byte) { // open file file, err := os.OpenFile("/dev/sdc", os.O_WRONLY|os.O_CREATE|syscall.O_DIRECT, 0666) if err != nil { fmt.Printf("An error occurred with file opening or creation\n") return } defer file.Close() // write file fmt.Println("buffer ", unsafe.Pointer(&buf)) fmt.Println("buffer[0] ", unsafe.Pointer(&buf[0])) buf2 := buf[4:516] fmt.Println("write with buffer ", unsafe.Pointer(&buf2[0])) _, err = file.WriteAt(buf2, 512) if err != nil { fmt.Println("write error ", err) } else { fmt.Println("write succeed") } } 这段代码运行后,我们可以看到如下结果。 buffer 0xc42000a2e0 buffer[0] 0xc42005a000 write with buffer 0xc42005a004 write error write /dev/sdc: invalid argument 看到了write error,WriteAt函数这次的返回值给出的并不是nil了,我们的写入操作失败了,其返回值返回了一个不可理解的invalid argument(非法参数)。 but我们的参数毫无问题啊!下面我们尝试一下把要写入的数据改为512对齐。 func writeWithAlignmentWithDIRECT(buf []byte) { // open file file, err := os.OpenFile("/dev/sdd", os.O_WRONLY|os.O_CREATE|syscall.O_DIRECT, 0666) if err != nil { fmt.Printf("An error occurred with file opening or creation\n") return } defer file.Close() // write file fmt.Println("buffer ", unsafe.Pointer(&buf)) fmt.Println("buffer[0] ", unsafe.Pointer(&buf[0])) buf2 := buf[512 : 512+512] fmt.Println("write with buffer ", unsafe.Pointer(&buf2[0])) _, err = file.WriteAt(buf2, 512) if err != nil { fmt.Println("write error ", err) } else { fmt.Println("write succeed") } } 这段代码运行后,结果如下。 white with alignment and DIRECT: buffer 0xc42000a340 buffer[0] 0xc42005a000 write with buffer 0xc42005a200 write succeed 我们的写操作成功了!而这段代码与上次未成功的不同之处只有一个,那就是将要写入数据的首地址改成了512对齐。 通过这三段go程序,我们很清晰的验证了绕过page cache写文件的条件。 类似的,下面给出验证绕过page cache读文件也需要512对齐条件的代码。 func readWithoutAlignmentWithoutDIRECT(buf []byte) { // read file file, err := os.OpenFile("/dev/sdb", os.O_RDONLY, 0666) if err != nil { fmt.Printf("An error occurred whit file ipening.\n") return } defer file.Close() buf = buf[2:514] fmt.Println("read with buffer ", unsafe.Pointer(&buf[0])) _, err = file.ReadAt(buf, 512) if err != nil { fmt.Println("read error ", err) } else { fmt.Println("read succeed", buf) } } func readWithoutAlignmentWithDIRECT(buf []byte) { // read file file, err := os.OpenFile("/dev/sdc", os.O_RDONLY|syscall.O_DIRECT, 0666) if err != nil { fmt.Printf("An error occurred whit file ipening.\n") return } defer file.Close() buf = buf[2:514] fmt.Println("read with buffer ", unsafe.Pointer(&buf[0])) _, err = file.ReadAt(buf, 512) if err != nil { fmt.Println("read error ", err) } else { fmt.Println("read succeed", buf) } } func readWithAlignmentWithDIRECT(buf []byte) { // read file file, err := os.OpenFile("/dev/sdd", os.O_RDONLY|syscall.O_DIRECT, 0666) if err != nil { fmt.Printf("An error occurred whit file ipening.\n") return } defer file.Close() buf = buf[512 : 512+512] fmt.Println("read with buffer ", unsafe.Pointer(&buf[0])) _, err = file.ReadAt(buf, 512) if err != nil { fmt.Println("read error ", err) } else { fmt.Println("read succeed", buf) } } 这三个函数的运行结果分如下。 read with buffer 0xc42005a002 read succeed [4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
read with buffer 0xc42005a002 read error read /dev/sdc: invalid argument
read with buffer 0xc42005a200 read succeed [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] 可以看出,由于最初我们将切片的前20位分别赋值为0-20,其他位赋值默认值为0,第一个写入函数将buf[4:516]写入到/dev/sdb中,第二个写入函数写入失败,第三个写入函数将buf[512 : 512+512]写入到/dev/sdd,所以根据读取结果可以看出,我们的读取函数也是ok的。 最后,给出整段测试程序的完整代码。 package main import ( "fmt" "os" "syscall" "unsafe" ) func main() { buf := make([]byte, 8192) for i := 0; i < 20; i++ { buf[i] = byte(i) } fmt.Println("----------------------------------------") fmt.Println("white without alignment and DIRECT:") writeWithoutAlignmentWithoutDIRECT(buf) fmt.Println("----------------------------------------") fmt.Println("white without alignment but with DIRECT:") writeWithoutAlignmentWithDIRECT(buf) fmt.Println("----------------------------------------") fmt.Println("white with alignment and DIRECT:") writeWithAlignmentWithDIRECT(buf) fmt.Println("----------------------------------------") fmt.Println("read without alignment and DIRECT:") readWithoutAlignmentWithoutDIRECT(buf) fmt.Println("----------------------------------------") fmt.Println("read without alignment but with DIRECT:") readWithoutAlignmentWithDIRECT(buf) fmt.Println("----------------------------------------") fmt.Println("read with alignment and DIRECT:") readWithAlignmentWithDIRECT(buf) } func writeWithoutAlignmentWithoutDIRECT(buf []byte) { // open file file, err := os.OpenFile("/dev/sdb", os.O_WRONLY|os.O_CREATE, 0666) if err != nil { fmt.Printf("An error occurred with file opening or creation\n") return } defer file.Close() // write file fmt.Println("buffer ", unsafe.Pointer(&buf)) fmt.Println("buffer[0] ", unsafe.Pointer(&buf[0])) buf2 := buf[4:516] fmt.Println("write with buffer ", unsafe.Pointer(&buf2[0])) _, err = file.WriteAt(buf2, 512) if err != nil { fmt.Println("write error ", err) } else { fmt.Println("write succeed") } } func writeWithoutAlignmentWithDIRECT(buf []byte) { // open file file, err := os.OpenFile("/dev/sdc", os.O_WRONLY|os.O_CREATE|syscall.O_DIRECT, 0666) if err != nil { fmt.Printf("An error occurred with file opening or creation\n") return } defer file.Close() // write file fmt.Println("buffer ", unsafe.Pointer(&buf)) fmt.Println("buffer[0] ", unsafe.Pointer(&buf[0])) buf2 := buf[4:516] fmt.Println("write with buffer ", unsafe.Pointer(&buf2[0])) _, err = file.WriteAt(buf2, 512) if err != nil { fmt.Println("write error ", err) } else { fmt.Println("write succeed") } } func writeWithAlignmentWithDIRECT(buf []byte) { // open file file, err := os.OpenFile("/dev/sdd", os.O_WRONLY|os.O_CREATE|syscall.O_DIRECT, 0666) if err != nil { fmt.Printf("An error occurred with file opening or creation\n") return } defer file.Close() // write file fmt.Println("buffer ", unsafe.Pointer(&buf)) fmt.Println("buffer[0] ", unsafe.Pointer(&buf[0])) buf2 := buf[512 : 512+512] fmt.Println("write with buffer ", unsafe.Pointer(&buf2[0])) _, err = file.WriteAt(buf2, 512) if err != nil { fmt.Println("write error ", err) } else { fmt.Println("write succeed") } } func readWithoutAlignmentWithoutDIRECT(buf []byte) { // read file file, err := os.OpenFile("/dev/sdb", os.O_RDONLY, 0666) if err != nil { fmt.Printf("An error occurred whit file ipening.\n") return } defer file.Close() buf = buf[2:514] fmt.Println("read with buffer ", unsafe.Pointer(&buf[0])) _, err = file.ReadAt(buf, 512) if err != nil { fmt.Println("read error ", err) } else { fmt.Println("read succeed", buf) } } func readWithoutAlignmentWithDIRECT(buf []byte) { // read file file, err := os.OpenFile("/dev/sdc", os.O_RDONLY|syscall.O_DIRECT, 0666) if err != nil { fmt.Printf("An error occurred whit file ipening.\n") return } defer file.Close() buf = buf[2:514] fmt.Println("read with buffer ", unsafe.Pointer(&buf[0])) _, err = file.ReadAt(buf, 512) if err != nil { fmt.Println("read error ", err) } else { fmt.Println("read succeed", buf) } } func readWithAlignmentWithDIRECT(buf []byte) { // read file file, err := os.OpenFile("/dev/sdd", os.O_RDONLY|syscall.O_DIRECT, 0666) if err != nil { fmt.Printf("An error occurred whit file ipening.\n") return } defer file.Close() buf = buf[512 : 512+512] fmt.Println("read with buffer ", unsafe.Pointer(&buf[0])) _, err = file.ReadAt(buf, 512) if err != nil { fmt.Println("read error ", err) } else { fmt.Println("read succeed", buf) } }
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