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Adjust block size used by dd to speed QEMU domain save operations.

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See https://bugzilla.redhat.com/show_bug.cgi?id=599091

Saving a paused 512MB domain took 3m47s with the old block size of 512
bytes. Changing the block size to 1024*1024 decreased the time to 56
seconds. (Doubling again to 2048*1024 yielded 0 improvement; lowering
to 512k increased the save time to 1m10s, about 20%)
This commit is contained in:
Laine Stump 2010-06-03 23:36:00 -04:00
parent 1d45e1b622
commit 20206a4bc9
2 changed files with 13 additions and 2 deletions
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10
src/qemu/qemu_driver.c
View File

@ -4964,7 +4964,15 @@ static int qemudDomainSaveFlag(virDomainPtr dom, const char *path,
/* Due to way we append QEMU state on our header with dd, /* Due to way we append QEMU state on our header with dd,
* we need to ensure there's a 512 byte boundary. Unfortunately * we need to ensure there's a 512 byte boundary. Unfortunately
* we don't have an explicit offset in the header, so we fake * we don't have an explicit offset in the header, so we fake
* it by padding the XML string with NULLs */ * 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) { if (offset % QEMU_MONITOR_MIGRATE_TO_FILE_BS) {
unsigned long long pad = unsigned long long pad =
QEMU_MONITOR_MIGRATE_TO_FILE_BS - QEMU_MONITOR_MIGRATE_TO_FILE_BS -

5
src/qemu/qemu_monitor.h
View File

@ -261,7 +261,10 @@ int qemuMonitorMigrateToCommand(qemuMonitorPtr mon,
unsigned int background, unsigned int background,
const char * const *argv); const char * const *argv);
# define QEMU_MONITOR_MIGRATE_TO_FILE_BS 512llu /* In general, a larger BS means better domain save performance,
* at the expense of a larger resulting file - see qemu_driver.c
*/
# define QEMU_MONITOR_MIGRATE_TO_FILE_BS (1024llu * 1024)
int qemuMonitorMigrateToFile(qemuMonitorPtr mon, int qemuMonitorMigrateToFile(qemuMonitorPtr mon,
unsigned int background, unsigned int background,