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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

Daniel
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