使用go語言繞過page cache讀寫檔案

標籤：賦值   return   file   看到了   port   問題   Go語言   pointer   地址   

有時候，我們希望我們的資料不通過page cache的緩衝直接落盤。go語言裡，用參數DIRECT開啟檔案可以實現這一點要求。

但這樣做有一個硬性的要求，就是在讀寫的時候，對應的資料在記憶體中的地址一定要滿足512對齊，即首地址的2進位形式中後面至少要有9個0結尾，且資料長度為512位元組的整數倍，否則讀寫會失敗。

我們用go語言中的切片slice來驗證這件事。

首先我們建立一個go語言的切片並隨便賦一些值：

buf := make([]byte, 8192)for i := 0; i < 20; i++ {buf[i] = byte(i)}

我們首先嘗試一下正常的讀寫檔案過程，先不使用DIRECT參數繞開page cache。

func writeWithoutAlignmentWithoutDIRECT(buf []byte) {// open filefile, 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 filefmt.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")}}

這段代碼的運行結果如下：

buffer  0xc42000a2a0buffer[0]  0xc42005a000write with buffer  0xc42005a004write succeed

可以看出，這個切片的地址是0xc42000a2a0，這個切片內資料的首地址是0xc42005a000，這是一個與c語言不同的地方，c語言的資料首地址即為其中資料的首地址，而go語言中，切片的地址和切片內資料首地址是不同的。

我們要寫入的資料的首地址是從切片的第5個元素開始，其首地址為0xc42005a004，雖然並沒有512對齊，但是由於我們沒有嘗試繞過page cache，所以根據WriteAt函數的傳回值err可以看到，我們的寫入操作是成功的。

下面我們嘗試一下使用DIRECT參數開啟檔案，繞過page cache進行寫資料操作，其他參數不變。

func writeWithoutAlignmentWithDIRECT(buf []byte) {// open filefile, 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 filefmt.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")}}

這段代碼運行後，我們可以看到如下結果。

buffer  0xc42000a2e0buffer[0]  0xc42005a000write with buffer  0xc42005a004write error  write /dev/sdc: invalid argument

看到了write error，WriteAt函數這次的傳回值給出的並不是nil了，我們的寫入操作失敗了，其傳回值返回了一個不可理解的invalid argument（非法參數）。

but我們的參數毫無問題啊！下面我們嘗試一下把要寫入的資料改為512對齊。

func writeWithAlignmentWithDIRECT(buf []byte) {// open filefile, 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 filefmt.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")}}

這段代碼運行後，結果如下。

white with alignment and DIRECT:buffer  0xc42000a340buffer[0]  0xc42005a000write with buffer  0xc42005a200write succeed

我們的寫操作成功了！而這段代碼與上次未成功的不同之處只有一個，那就是將要寫入資料的首地址改成了512對齊。

通過這三段go程式，我們很清晰的驗證了繞過page cache寫檔案的條件。

類似的，下面給出驗證繞過page cache讀檔案也需要512對齊條件的代碼。

func readWithoutAlignmentWithoutDIRECT(buf []byte) {// read filefile, 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 filefile, 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 filefile, 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)}}

這三個函數的運行結果分如下。

read with buffer  0xc42005a002read 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  0xc42005a002read error  read /dev/sdc: invalid argument

　　

read with buffer  0xc42005a200read 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，由於最初我們將切片的前20位分別賦值為0-20，其他位賦值預設值為0，第一個寫入函數將buf[4:516]寫入到/dev/sdb中，第二個寫入函數寫入失敗，第三個寫入函數將buf[512 : 512+512]寫入到/dev/sdd，所以根據讀取結果可以看出，我們的讀取函數也是ok的。

最後，給出整段測試程式的完整代碼。

package mainimport ("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 filefile, 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 filefmt.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 filefile, 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 filefmt.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 filefile, 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 filefmt.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 filefile, 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 filefile, 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 filefile, 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)}}

　　

使用go語言繞過page cache讀寫檔案