Re: [Libguestfs] [nbdkit PATCH] sh: Prefer dd bs=1 over iflag=count_bytes
On 8/18/20 9:58 AM, Richard W.M. Jones wrote:
On Mon, Aug 17, 2020 at 11:35:39AM -0500, Eric Blake wrote:
> While iflag=count_bytes combined with bs > 1 allows for more efficient
> operation, it is a feature of GNU dd, and not present on other
> implementations such as BSD. Sticking to just POSIX features makes
> things more portable.
I'm not very convinced by this. Maybe we should persuade
the BSD folk to implement this useful feature instead?
Wouldn't hurt to try.
As one example:
> diff --git a/docs/nbdkit-loop.pod b/docs/nbdkit-loop.pod
> index 055b5750..b21c2212 100644
> --- a/docs/nbdkit-loop.pod
> +++ b/docs/nbdkit-loop.pod
> @@ -120,7 +120,7 @@ creates a disk which contains a bad sector:
> echo EIO Bad block >&2
> exit 1
> else
> - dd if=/dev/zero count=$3 iflag=count_bytes
> + dd bs=1 if=/dev/zero count=$3
> fi ;;
> *) exit 2 ;;
> esac
this change could be a killer for performance since the suggested size
of the disk is 64M.
Another thing we could do: I've got a proposal out to let plugins
advertise block size preferences. Once that is in place, a shell script
that uses dd could advertise minimum block size of 512, leave bs=512
(the default), and compute count/skip/seek by $(($3 / 512)) when the
script must be portable to BSD dd. Performance will then not be
penalized, at the expense that the user would have to use the
--filter=blocksize to get sub-block read/writes.
But yeah, given the definite slowdown to the testsuite for
heavy-hitters, I'm thinking the best we can do without a minimum block
size advertisement is documentation of why we prefer GNU dd features,
and the slower fallback possible on BSD.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org