7:31 a.m.
Followup to https://bugzilla.redhat.com/show_bug.cgi?id=599091,
commit 20206a4b, to reduce disk waste in padding.
* src/qemu/qemu_monitor.h (QEMU_MONITOR_MIGRATE_TO_FILE_BS): Drop
back to 512.
(QEMU_MONITOR_MIGRATE_TO_FILE_TRANSFER_SIZE): New macro.
* src/qemu/qemu_driver.c (qemudDomainSaveFlag): Update comment.
* src/qemu/qemu_monitor_text.c (qemuMonitorTextMigrateToFile): Use
two invocations of dd to output non-aligned large blocks.
* src/qemu/qemu_monitor_json.c (qemuMonitorJSONMigrateToFile):
Likewise.
---
v2: avoid truncating regular file destination
v3: change minimum block size from 512 to 4k, to avoid unaligned
RMW cycles on newer disks with 4k sectors
src/qemu/qemu_driver.c | 7 -------
src/qemu/qemu_monitor.h | 14 ++++++++++----
src/qemu/qemu_monitor_json.c | 14 ++++++++++----
src/qemu/qemu_monitor_text.c | 14 ++++++++++----
4 files changed, 30 insertions(+), 19 deletions(-)
diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index bc8dcfa..4edf5ec 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -5000,13 +5000,6 @@ static int qemudDomainSaveFlag(virDomainPtr dom, const char *path,
* we need to ensure there's a 512 byte boundary. Unfortunately
* we don't have an explicit offset in the header, so we fake
* it by padding the XML string with NULLs.
- *
- * XXX: This means there will be (QEMU_MONITOR_MIGRATE_TO_FILE_BS
- * - strlen(xml)) bytes of wastage in each file.
- * Unfortunately, a large BS is needed for reasonable
- * performance. It would be nice to find a replacement for dd
- * that could specify the start offset in bytes rather than
- * blocks, to eliminate this waste.
*/
if (offset % QEMU_MONITOR_MIGRATE_TO_FILE_BS) {
unsigned long long pad =
diff --git a/src/qemu/qemu_monitor.h b/src/qemu/qemu_monitor.h
index e03b4df..6c104f1 100644
--- a/src/qemu/qemu_monitor.h
+++ b/src/qemu/qemu_monitor.h
@@ -1,7 +1,7 @@
/*
* qemu_monitor.h: interaction with QEMU monitor console
*
- * Copyright (C) 2006-2009 Red Hat, Inc.
+ * Copyright (C) 2006-2010 Red Hat, Inc.
* Copyright (C) 2006 Daniel P. Berrange
*
* This library is free software; you can redistribute it and/or
@@ -261,10 +261,16 @@ int qemuMonitorMigrateToCommand(qemuMonitorPtr mon,
unsigned int background,
const char * const *argv);
-/* In general, a larger BS means better domain save performance,
- * at the expense of a larger resulting file - see qemu_driver.c
+/* In general, BS is the smallest fundamental block size we can use to
+ * access a block device; everything must be aligned to a multiple of
+ * this. Linux generally supports a BS as small as 512, but with
+ * newer disks with 4k sectors, performance is better if we guarantee
+ * alignment to the sector size. However, operating on BS-sized
+ * blocks is painfully slow, so we also have a transfer size that is
+ * larger but only aligned to the smaller block size.
*/
-# define QEMU_MONITOR_MIGRATE_TO_FILE_BS (1024llu * 1024)
+# define QEMU_MONITOR_MIGRATE_TO_FILE_BS (1024llu * 4)
+# define QEMU_MONITOR_MIGRATE_TO_FILE_TRANSFER_SIZE (1024llu * 1024)
int qemuMonitorMigrateToFile(qemuMonitorPtr mon,
unsigned int background,
diff --git a/src/qemu/qemu_monitor_json.c b/src/qemu/qemu_monitor_json.c
index e89ad43..40218aa 100644
--- a/src/qemu/qemu_monitor_json.c
+++ b/src/qemu/qemu_monitor_json.c
@@ -1695,10 +1695,16 @@ int qemuMonitorJSONMigrateToFile(qemuMonitorPtr mon,
goto cleanup;
}
- if (virAsprintf(&dest, "exec:%s | dd of=%s bs=%llu seek=%llu",
- argstr, safe_target,
- QEMU_MONITOR_MIGRATE_TO_FILE_BS,
- offset / QEMU_MONITOR_MIGRATE_TO_FILE_BS) < 0) {
+ /* Two dd processes, sharing the same stdout, are necessary to
+ * allow starting at an alignment of 512, but without wasting
+ * padding to get to the larger alignment useful for speed. Use
+ * <> redirection to avoid truncating a regular file. */
+ if (virAsprintf(&dest, "exec:%s | { dd bs=%llu seek=%llu if=/dev/null
&& "
+ "dd bs=%llu; } 1<>%s",
+ argstr, QEMU_MONITOR_MIGRATE_TO_FILE_BS,
+ offset / QEMU_MONITOR_MIGRATE_TO_FILE_BS,
+ 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 c0ebe5f..c086962 100644
--- a/src/qemu/qemu_monitor_text.c
+++ b/src/qemu/qemu_monitor_text.c
@@ -1273,10 +1273,16 @@ int qemuMonitorTextMigrateToFile(qemuMonitorPtr mon,
goto cleanup;
}
- if (virAsprintf(&dest, "exec:%s | dd of=%s bs=%llu seek=%llu",
- argstr, safe_target,
- QEMU_MONITOR_MIGRATE_TO_FILE_BS,
- offset / QEMU_MONITOR_MIGRATE_TO_FILE_BS) < 0) {
+ /* Two dd processes, sharing the same stdout, are necessary to
+ * allow starting at an alignment of 512, but without wasting
+ * padding to get to the larger alignment useful for speed. Use
+ * <> redirection to avoid truncating a regular file. */
+ if (virAsprintf(&dest, "exec:%s | { dd bs=%llu seek=%llu if=/dev/null
&& "
+ "dd bs=%llu; } 1<>%s",
+ argstr, QEMU_MONITOR_MIGRATE_TO_FILE_BS,
+ offset / QEMU_MONITOR_MIGRATE_TO_FILE_BS,
+ QEMU_MONITOR_MIGRATE_TO_FILE_TRANSFER_SIZE,
+ safe_target) < 0) {
virReportOOMError();
goto cleanup;
}
--
1.7.0.1