[Libguestfs] [PATCH 2/2] Discard unwritten ranges

[Pádraig Brady]

On 10/22/2013 05:57 PM, Gabriel de Perthuis wrote:
> ---
>  pxzcat.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/pxzcat.c b/pxzcat.c
> index 9bcdc36..55ccfc0 100644
> --- a/pxzcat.c
> +++ b/pxzcat.c
> @@ -44,10 +44,11 @@
>  #include <sys/types.h>
>  #include <error.h>
>  #include <errno.h>
>  #include <getopt.h>
>  #include <pthread.h>
> +#include <linux/falloc.h>
>   #include <lzma.h>
>   #define DEBUG 0
>  @@ -145,10 +146,11 @@ usage (int exitcode)
>  static void
>  xzfile_uncompress (const char *filename, const char *outputfile,
>                     unsigned nr_threads)
>  {
>    int fd, ofd;
> +  off_t hole_start, data_start;
>    uint64_t size;
>    lzma_index *idx;
>     /* Open the file. */
>    fd = open (filename, O_RDONLY);
> @@ -176,10 +178,29 @@ xzfile_uncompress (const char *filename, const
> char *outputfile,
>    posix_fadvise (fd, 0, 0, POSIX_FADV_RANDOM|POSIX_FADV_DONTNEED);
>     /* Iterate over blocks. */
>    iter_blocks (idx, nr_threads, filename, fd, outputfile, ofd);
>  +  /* discard ranges that were allocated but not written */
> +  data_start = 0;
> +  while (data_start < size) {
> +    hole_start = lseek (ofd, data_start, SEEK_HOLE);
> +    if (hole_start == (off_t) -1)
> +      error (EXIT_FAILURE, errno, "lseek: %s", outputfile);
> +    if (hole_start == size)
> +      break;
> +    data_start = lseek (ofd, hole_start, SEEK_DATA);
> +    if (data_start == (off_t) -1) {
> +      if (errno == ENXIO)
> +        data_start = size;
> +      else
> +        error (EXIT_FAILURE, errno, "lseek: %s", outputfile);
> +    }
> +    if (fallocate (ofd, FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE,
> hole_start, data_start - hole_start) == -1)

FALLOC_FL_PUNCH_HOLE is new. So you might want to:
#ifdef FALLOC_FL_PUNCH_HOLE
fallocate(....)
#endif

You may want to avoid the original fallocate() too,
if PUNCH_HOLE wasn't available.

cheers,
Pádraig.