QEMU defaults to allowing the host OS to cache all disk I/O. THis has a
couple of problems
- It is a waste of memory because the guest already caches I/O ops
- It is unsafe on host OS crash - all unflushed guest I/O will be
lost, and there's no ordering guarentees, so metadata updates could
be flushe to disk, while the journal updates were not. Say goodbye
to your filesystem.
- It makes benchmarking more or less impossible / worthless because
what the benchmark things are disk writes just sit around in memory
so guest disk performance appears to exceed host diskperformance.
This patch disables caching on all QEMU guests. NB, Xen has long done this
for both PV & HVM guests - QEMU only gained this ability when -drive was
introduced, and sadly kept the default to unsafe cache=on settings.
Daniel
diff -r 4a0ccc9dc530 src/qemu_conf.c
--- a/src/qemu_conf.c Wed Oct 08 11:53:45 2008 +0100
+++ b/src/qemu_conf.c Wed Oct 08 11:59:33 2008 +0100
@@ -460,6 +460,8 @@
flags |= QEMUD_CMD_FLAG_DRIVE;
if (strstr(help, "boot=on"))
flags |= QEMUD_CMD_FLAG_DRIVE_BOOT;
+ if (strstr(help, "cache=on"))
+ flags |= QEMUD_CMD_FLAG_DRIVE_CACHE;
if (version >= 9000)
flags |= QEMUD_CMD_FLAG_VNC_COLON;
@@ -959,13 +961,15 @@
break;
}
- snprintf(opt, PATH_MAX, "file=%s,if=%s,%sindex=%d%s",
+ snprintf(opt, PATH_MAX, "file=%s,if=%s,%sindex=%d%s%s",
disk->src ? disk->src : "", bus,
media ? media : "",
idx,
bootable &&
disk->device == VIR_DOMAIN_DISK_DEVICE_DISK
- ? ",boot=on" : "");
+ ? ",boot=on" : "",
+ qemuCmdFlags & QEMUD_CMD_FLAG_DRIVE_BOOT
+ ? ",cache=off" : "");
ADD_ARG_LIT("-drive");
ADD_ARG_LIT(opt);
diff -r 4a0ccc9dc530 src/qemu_conf.h
--- a/src/qemu_conf.h Wed Oct 08 11:53:45 2008 +0100
+++ b/src/qemu_conf.h Wed Oct 08 11:59:33 2008 +0100
@@ -44,7 +44,8 @@
QEMUD_CMD_FLAG_NO_REBOOT = (1 << 2),
QEMUD_CMD_FLAG_DRIVE = (1 << 3),
QEMUD_CMD_FLAG_DRIVE_BOOT = (1 << 4),
- QEMUD_CMD_FLAG_NAME = (1 << 5),
+ QEMUD_CMD_FLAG_DRIVE_CACHE = (1 << 5),
+ QEMUD_CMD_FLAG_NAME = (1 << 6),
};
/* Main driver state */
--
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