Re: [libvirt] [PATCH] qemu: reduce file padding requirements

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).