On Thu, Jun 10, 2010 at 05:03:10AM -0600, Eric Blake wrote:
> On 06/10/2010 04:16 AM, Daniel P. Berrange wrote:
> >>
> >> - if (virAsprintf(&dest, "exec:%s | dd of=%s bs=%llu seek=%llu",
> >> - argstr, safe_target,
> >> - QEMU_MONITOR_MIGRATE_TO_FILE_BS,
> >> - offset / QEMU_MONITOR_MIGRATE_TO_FILE_BS) < 0) {
> >> + /* Two dd processes, sharing the same stdout, are necessary to
> >> + * allow starting at an alignment of 512, but without wasting
> >> + * padding to get to the larger alignment useful for speed. */
> >> + if (virAsprintf(&dest, "exec:%s | { dd bs=%llu seek=%llu if=/dev/null && "
> >> + "dd bs=%llu; } >%s",
> >
> > Does this work with block devices as the target ? We previously
> > switched from cat>> to dd, because it didn't work correctly with block
> > devs.
>
> Yes - the problem with >>dev was that it appended (which doesn't make
> sense for a block device); while >dev opens a seekable fd. One
> difference between 'dd >file' and 'dd of=file' is which process opens
> the device; but to share the fd between two dd invocations, we have to
> open the file in the shell rather than with of=file. The other
> difference is that >file truncates; for block devices, truncation is a
> no-op, but for regular files, this wipes out any pre-existing contents
> (such as the header we are skipping over) - is that an issue? If so,
> then we should use <>file instead of >file, to open a read-write fd
> without forcing truncation (even if we only use the fd for writing).
Yes, we need to avoid truncation - the seek is there to skip over the
metadata header, so we can't truncate that
Daniel
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