qemu-img: Require larger zero areas for sparse handling

By default, require 4k of consecutive zero bytes for qemu-img to make the
output file sparse by not issuing a write request for the zeroed parts. Add an
-S option to allow users to tune this setting.

This helps to avoid situations where a lot of zero sectors and data sectors are
mixed and qemu-img tended to issue many tiny 512 byte writes.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/qemu-img.texi b/qemu-img.texi
index 495a1b6695..70fa321dff 100644
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -40,6 +40,11 @@ indicates that target image must be compressed (qcow format only)
 with or without a command shows help and lists the supported formats
 @item -p
 display progress bar (convert and rebase commands only)
+@item -S @var{size}
+indicates the consecutive number of bytes that must contain only zeros
+for qemu-img to create a sparse image during conversion. This value is rounded
+down to the nearest 512 bytes. You may use the common size suffixes like
+@code{k} for kilobytes.
 @end table
 
 Parameters to snapshot subcommand:
@@ -86,7 +91,7 @@ it doesn't need to be specified separately in this case.
 
 Commit the changes recorded in @var{filename} in its base image.
 
-@item convert [-c] [-p] [-f @var{fmt}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] @var{filename} [@var{filename2} [...]] @var{output_filename}
+@item convert [-c] [-p] [-f @var{fmt}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_name}] [-S @var{sparse_size}] @var{filename} [@var{filename2} [...]] @var{output_filename}
 
 Convert the disk image @var{filename} or a snapshot @var{snapshot_name} to disk image @var{output_filename}
 using format @var{output_fmt}. It can be optionally compressed (@code{-c}