Fallocate System Call

by Bramante - - | Comments

嵌入式系統的儲存空間相當有限, 如果在下載檔案前, 先把檔案按預期的大小給建立起來, 下載時再來寫入真正的檔案內容, 這樣便能保證不會發生下載到一半卻出現Disk Full的狀況.

Linux系統有個fallocate system call可以讓我們快速建立內容全為0x00的檔案, 相當適合用來處理這樣的問題.

以下的程式會利用fallocate system call, 建立多個1GB大小的file, 直到無法再寫檔為止:

test.c
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#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <ctype.h>
#include <linux/falloc.h>

int main(int argc, char **argv)
{
 FILE* fh ;
 char fname[128] ;
 loff_t  length = 1024*1024*1024 ;
 loff_t  offset = 0 ;
 int falloc_mode = 0 ;
 int error;
 int i ;
 for( i = 0 ; i < 200 ; i ++ )
    {
     sprintf(fname,"./data%04d",i);
     fh = fopen( fname,"w+");
     if(fh)
       {
        error = syscall(SYS_fallocate, fileno(fh), falloc_mode, offset, length);
        if(error < 0)
          {
           printf("[*][error] no space !!!\n");
           perror("fallocate failed");
           exit(EXIT_FAILURE);
          }
        fclose(fh);
       }
    }
 return 0;
}

編譯與執行:

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bramante@matrix:~/test/fallocate$ gcc -o ./test ./test.c
bramante@matrix:~/test/fallocate$ time ./test
[*][error] no space !!!
fallocate failed: Disk quota exceeded

real    0m0.897s
user    0m0.000s
sys     0m0.100s
bramante@matrix:~/test/fallocate$ ll
total 92309096
drwxrwxr-x 2 bramante bramante       4096 Sep  8 19:04 ./
drwxr-xr-x 6 bramante bramante       4096 Sep  8 18:16 ../
-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0000
-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0001
-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0002
-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0003
-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0004
-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0005
-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0006
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-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0008
-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0009
-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0010
-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0011
-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0012
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-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0020
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-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0034
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-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0070
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-rw-rw-r-- 1 bramante bramante 1073741824 Sep  8 19:04 data0087
-rw-rw-r-- 1 bramante bramante   34848768 Sep  8 19:04 data0088
-rwxrwxr-x 1 bramante bramante       8799 Sep  8 19:04 test*
-rw-rw-r-- 1 bramante bramante        721 Sep  8 19:04 test.c
bramante@matrix:~/test/fallocate$

不到1 sec可以建立出超過88 GB的檔案, 真的是好快, 而且disk quota用盡時, 程式還能順利停下來, 實在是很不錯.

從以下的執行範例, 可以看出建檔與砍檔都很快, 都不用1 sec, 而且檔案的確有內容, 內容都為0x00 :

Terminal
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bramante@matrix:~/test/fallocate$ time ./test
[*][error] no space !!!
fallocate failed: Disk quota exceeded

real    0m0.741s
user    0m0.000s
sys     0m0.100s
bramante@matrix:~/test/fallocate$ hexdump -C data0087
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
40000000
bramante@matrix:~/test/fallocate$ hexdump -C data0088
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
0213a000
bramante@matrix:~/test/fallocate$ hexdump -C data0001
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
40000000
bramante@matrix:~/test/fallocate$ time rm data0*

real    0m0.864s
user    0m0.004s
sys     0m0.820s
bramante@matrix:~/test/fallocate$

另外要提一下, fallocate其實也可以用來把file裡的內容給刪掉一段, 例如, 刪除file開頭的4096 byte:

syscall(SYS_fallocate, fileno(fh), FALLOC_FL_COLLAPSE_RANGE, 0, 4096);

這個功能滿強大的, 如果能用的話, 刪除檔案裡一些不需要的內容會很快, 只可惜我的Linux server太舊了, 這個功能沒有試成功.

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