[Libguestfs] [nbdkit PATCH] file: Add trim support

[Richard W.M. Jones]

On Tue, Jan 30, 2018 at 08:14:05PM -0600, Eric Blake wrote:
> We already have support in the file driver for punching holes
> during .zero with the may_trim flag (via FALLOC_FL_PUNCH_HOLE),
> so we should use the same mechanism to support .trim.  Note that
> the NBD spec says that trim is advisory (we can return success
> even if we did nothing); but at the same time, it's nicer to
> avoid advertising the feature if we know for sure we can't do
> it, so we also need .can_trim.  Note that there's still an
> element of runtime behavior, as FALLOC_FL_PUNCH_HOLE doesn't
> work on all filesystems; there we fall back on the NBD
> protocol allowing us to be advisory for all but a handful of
> errno values that we can directly report back over NBD.
> 
> Signed-off-by: Eric Blake <eblake at redhat.com>
> ---
> 
> I couldn't test --filter=log results on trim commands without
> at least one plugin that supports trim ;)
> 
>  plugins/file/file.c | 47 ++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 44 insertions(+), 3 deletions(-)
> 
> diff --git a/plugins/file/file.c b/plugins/file/file.c
> index 1fe4191..081848b 100644
> --- a/plugins/file/file.c
> +++ b/plugins/file/file.c
> @@ -1,5 +1,5 @@
>  /* nbdkit
> - * Copyright (C) 2013 Red Hat Inc.
> + * Copyright (C) 2013-2018 Red Hat Inc.
>   * All rights reserved.
>   *
>   * Redistribution and use in source and binary forms, with or without
> @@ -175,6 +175,18 @@ file_get_size (void *handle)
>    return statbuf.st_size;
>  }
> 
> +static int
> +file_can_trim (void *handle)
> +{
> +  /* Trim is advisory, but we prefer to advertise it only when we can
> +   * actually (attempt to) punch holes.  */
> +#ifdef FALLOC_FL_PUNCH_HOLE
> +  return 1;
> +#else
> +  return 0;
> +#endif
> +}
> +
>  /* Read data from the file. */
>  static int
>  file_pread (void *handle, void *buf, uint32_t count, uint64_t offset)
> @@ -219,7 +231,7 @@ file_pwrite (void *handle, const void *buf, uint32_t count, uint64_t offset)
>    return 0;
>  }
> 
> -/* Write data to the file. */
> +/* Write zeroes to the file. */
>  static int
>  file_zero (void *handle, uint32_t count, uint64_t offset, int may_trim)
>  {
> @@ -268,6 +280,33 @@ file_flush (void *handle)
>    return 0;
>  }
> 
> +/* Punch a hole in the file. */
> +static int
> +file_trim (void *handle, uint32_t count, uint64_t offset)
> +{
> +  int r = -1;
> +#ifdef FALLOC_FL_PUNCH_HOLE
> +  struct handle *h = handle;
> +
> +  /* Trim is advisory; we don't care if it fails for anything other
> +   * than EIO or EPERM. */
> +  r = fallocate (h->fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> +                 offset, count);
> +  if (r < 0) {
> +    if (errno != EPERM && errno != EIO) {
> +      nbdkit_debug ("ignoring failed fallocate during trim: %m");
> +      r = 0;
> +    }
> +    else
> +      nbdkit_error ("fallocate: %m");
> +  }
> +#else
> +  /* Based on .can_trim, this should not be reached. */
> +  errno = EOPNOTSUPP;
> +#endif
> +  return r;
> +}
> +
>  static struct nbdkit_plugin plugin = {
>    .name              = "file",
>    .longname          = "nbdkit file plugin",
> @@ -279,10 +318,12 @@ static struct nbdkit_plugin plugin = {
>    .open              = file_open,
>    .close             = file_close,
>    .get_size          = file_get_size,
> +  .can_trim          = file_can_trim,
>    .pread             = file_pread,
>    .pwrite            = file_pwrite,
> -  .zero              = file_zero,
>    .flush             = file_flush,
> +  .trim              = file_trim,
> +  .zero              = file_zero,
>    .errno_is_preserved = 1,
>  };

ACK

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-top is 'top' for virtual machines.  Tiny program with many
powerful monitoring features, net stats, disk stats, logging, etc.
http://people.redhat.com/~rjones/virt-top