How to test if sparse file is supported

Question

Given a file descriptor or file name, how do I know if I can write to an arbitrary location without waiting for the intervening part be explicitly zeroed out on disk?

Answer 1

You can stat() the file to obtain file size and number of disk blocks, seek a relatively small number of disk blocks past the end of the file, write a known number of blocks, then stat the file again. Compare the original number of disk blocks to the final number. Just a few disk blocks shouldn't take too long to write if the file system doesn't support sparse file.

Given the original and final number of disk blocks, then try to determine if the file system supports sparse files. I say "try" because some filesystems can make this hard - for example, ZFS with compression enabled.

Something like this:

#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

int check( const char *filename )
{
    struct stat sb;
    long blocksize;
    off_t filesize;
    blkcnt_t origblocks;
    char *buffer;
    int fd;

    fd = open( filename, O_CREAT | O_RDWR, 0644 );

    fstat( fd, &sb );
    blocksize = sb.st_blksize;
    filesize = sb.st_size;
    origblocks = sb.st_blocks;

    lseek( fd, 16UL * blocksize, SEEK_END );

    buffer = malloc( blocksize );
    memset( buffer, 0xAA, blocksize );

    write( fd, buffer, blocksize );
    fsync( fd );

    free( buffer );

    // kludge to give ZFS time to update metadata
    for ( ;; )
    {
        stat( filename, &sb );
        if ( sb.st_blocks != origblocks )
        {
            break;
        }
    }

    printf( "file: %s\n filesystem: %s\n blocksize: %d\n size: %zd\n"
        " blocks: %zd\n orig blocks: %zd\n disk space: %zd\n",
        filename, sb.st_fstype, blocksize, sb.st_size,
        ( size_t ) sb.st_blocks, ( size_t ) origblocks,
        ( size_t ) ( 512UL * sb.st_blocks ) );

    // return file to original size
    ftruncate( fd, filesize );
    return( 0 );
}

int main( int argc, char **argv )
{
    for ( int ii = 1; ii < argc; ii++ )
    {
        check( argv[ ii ] );
    }

    return( 0 );
}

(error checking is omitted for clarity)

ZFS with compression enabled doesn't seem to update the file metadata quickly, hence the spinning waiting for the changes to appear.

When run on a Solaris 11 box with the files asdf (ZFS filesystem, compression enabled) /tmp/asdf (tmpfs file system), and /var/tmp/asdf (ZFS, no compression), that code produces the following output:

file: asdf
 filesystem: zfs
 blocksize: 131072
 size: 2228224
 blocks: 10
 orig blocks: 1
 disk space: 5120
file: /tmp/asdf
 filesystem: tmpfs
 blocksize: 4096
 size: 69632
 blocks: 136
 orig blocks: 0
 disk space: 69632
file: /var/tmp/asdf
 filesystem: zfs
 blocksize: 131072
 size: 2228224
 blocks: 257
 orig blocks: 1
 disk space: 131584

From that output, it should be obvious that /tmp/asdf is on a file system that doesn't support sparse files, and /var/tmp/asdf is in a file system that does support such files.

And plain asdf is on something else entirely, where writing 128 kB of data adds all of 9 512-byte disk blocks. From that, you can infer that there's some sort of compression going on in the filesystem. Offhand, I suspect it's pretty safe to assume any filesystem that supports such native compression is also going to support sparse files.

And the fastest way to determine if a filesystem supports sparse files when give a filename or open file descriptor is to call stat() on the filename or fstat() on the file descriptor, obtain the st_fstype field from the struct stat, and compare the file's filesystem type to a set of strings of filesystem types known to support sparse files.