Re: [libvirt] [PATCHv4 03/15] qemu: improve efficiency of dd during snapshots

On Wed, Mar 09, 2011 at 06:45:43PM -0700, Eric Blake wrote: > POSIX states about dd: > > If the bs=expr operand is specified and no conversions other than > sync, noerror, or notrunc are requested, the data returned from each > input block shall be written as a separate output block; if the read > returns less than a full block and the sync conversion is not > specified, the resulting output block shall be the same size as the > input block. If the bs=expr operand is not specified, or a conversion > other than sync, noerror, or notrunc is requested, the input shall be > processed and collected into full-sized output blocks until the end of > the input is reached. > > Since we aren't using conv=sync, there is no zero-padding, but our > use of bs= means that a short read results in a short write. If > instead we use ibs= and obs=, then short reads are collected and dd > only has to do a single write, which can make dd more efficient. > > * src/qemu/qemu_monitor.c (qemuMonitorMigrateToFile): > Avoid 'dd bs=', since it can cause short writes. > --- > src/qemu/qemu_monitor.c | 3 ++- > 1 files changed, 2 insertions(+), 1 deletions(-) ACK