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[libvirt] [PATCH 7/7] 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_text.c (qemuMonitorTextMigrateToFile):
Avoid 'dd bs=', since it can cause short writes.
* src/qemu/qemu_monitor_json.c (qemuMonitorJSONMigrateToFile):
Likewise.
---
 src/qemu/qemu_monitor_json.c |    5 +++--
 src/qemu/qemu_monitor_text.c |    5 +++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/qemu/qemu_monitor_json.c b/src/qemu/qemu_monitor_json.c
index 7387089..44f4fbc 100644
--- a/src/qemu/qemu_monitor_json.c
+++ b/src/qemu/qemu_monitor_json.c
@@ -1,7 +1,7 @@
 /*
  * qemu_monitor_json.c: interaction with QEMU monitor console
  *
- * Copyright (C) 2006-2010 Red Hat, Inc.
+ * Copyright (C) 2006-2011 Red Hat, Inc.
  * Copyright (C) 2006 Daniel P. Berrange
  *
  * This library is free software; you can redistribute it and/or
@@ -1751,10 +1751,11 @@ int qemuMonitorJSONMigrateToFile(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;
diff --git a/src/qemu/qemu_monitor_text.c b/src/qemu/qemu_monitor_text.c
index 291d958..bae71db 100644
--- a/src/qemu/qemu_monitor_text.c
+++ b/src/qemu/qemu_monitor_text.c
@@ -1,7 +1,7 @@
 /*
  * qemu_monitor_text.c: interaction with QEMU monitor console
  *
- * Copyright (C) 2006-2010 Red Hat, Inc.
+ * Copyright (C) 2006-2011 Red Hat, Inc.
  * Copyright (C) 2006 Daniel P. Berrange
  *
  * This library is free software; you can redistribute it and/or
@@ -1349,10 +1349,11 @@ int qemuMonitorTextMigrateToFile(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;
-- 
1.7.3.4


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