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

On 06/10/2010 04:16 AM, Daniel P. Berrange wrote: 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). And from the coreutils manual, 'info dd', this example validates the use of dual dd invocations on a block device (although it goes the opposite direction with a seek larger than the transfer size): Use different `dd' invocations to use different block sizes for skipping and I/O. For example, the following shell commands copy data in 512 KiB blocks between a disk and a tape, but do not save or restore a 4 KiB label at the start of the disk: disk=/dev/rdsk/c0t1d0s2 tape=/dev/rmt/0 # Copy all but the label from disk to tape. (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape # Copy from tape back to disk, but leave the disk label alone. (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk -- Eric Blake eblake(a)redhat.com +1-801-349-2682 Libvirt virtualization library http://libvirt.org