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

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(-) diff --git a/src/qemu/qemu_monitor.c b/src/qemu/qemu_monitor.c index eb6f8d4..765a1ae 100644 --- a/src/qemu/qemu_monitor.c +++ b/src/qemu/qemu_monitor.c @@ -1499,10 +1499,11 @@ int qemuMonitorMigrateToFile(qemuMonitorPtr mon, * <> redirection to avoid truncating a regular file. */ if (virAsprintf(&dest, "exec:" VIR_WRAPPER_SHELL_PREFIX "%s | " "{ dd bs=%llu seek=%llu if=/dev/null && " - "dd bs=%llu; } 1<>%s" VIR_WRAPPER_SHELL_SUFFIX, + "dd ibs=%llu obs=%llu; } 1<>%s" VIR_WRAPPER_SHELL_SUFFIX, argstr, QEMU_MONITOR_MIGRATE_TO_FILE_BS, offset / QEMU_MONITOR_MIGRATE_TO_FILE_BS, QEMU_MONITOR_MIGRATE_TO_FILE_TRANSFER_SIZE, + QEMU_MONITOR_MIGRATE_TO_FILE_TRANSFER_SIZE, safe_target) < 0) { virReportOOMError(); goto cleanup; -- 1.7.4