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Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk


From: Fabrice Bellard
Subject: Re: [Qemu-devel] [RFC][PATCH] make sure disk writes actually hit disk
Date: Sat, 29 Jul 2006 11:57:31 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.3) Gecko/20040913

Hi,

Using O_SYNC for disk image access is not acceptable: QEMU relies on the host OS to ensure that the data is written correctly. Even the current 'fsync' support is questionnable to say the least !

Please don't mix issues regarding QEMU disk handling and the underlying hypervisor/host OS block device handling.

Regards,

Fabrice.

Rik van Riel wrote:
This is the simple approach to making sure that disk writes actually
hit disk before we tell the guest OS that IO has completed.  Thanks
to DMA_MULTI_THREAD the performance still seems to be adequate.

A fancier solution would be to make the sync/non-sync behaviour of
the qemu disk backing store tunable from the guest OS, by tuning
the IDE disk write cache on/off with hdparm, and having hw/ide.c
call ->fsync functions in the block backends.

I'm willing to code up the fancy solution if people prefer that.


------------------------------------------------------------------------

Make sure disk writes really made it to disk before we report I/O
completion to the guest domain.  The DMA_MULTI_THREAD functionality
from the qemu-dm IDE emulation should make the performance overhead
of synchronous writes bearable, or at least comparable to native
hardware.

Signed-off-by: Rik van Riel <address@hidden>

--- xen-unstable-10712/tools/ioemu/block-bochs.c.osync  2006-07-28 
02:15:56.000000000 -0400
+++ xen-unstable-10712/tools/ioemu/block-bochs.c        2006-07-28 
02:21:08.000000000 -0400
@@ -91,7 +91,7 @@
     int fd, i;
     struct bochs_header bochs;
- fd = open(filename, O_RDWR | O_BINARY | O_LARGEFILE);
+    fd = open(filename, O_RDWR | O_BINARY | O_LARGEFILE | O_SYNC);
     if (fd < 0) {
         fd = open(filename, O_RDONLY | O_BINARY | O_LARGEFILE);
         if (fd < 0)
--- xen-unstable-10712/tools/ioemu/block.c.osync        2006-07-28 
02:15:56.000000000 -0400
+++ xen-unstable-10712/tools/ioemu/block.c      2006-07-28 02:19:27.000000000 
-0400
@@ -677,7 +677,7 @@
     int rv;
 #endif
- fd = open(filename, O_RDWR | O_BINARY | O_LARGEFILE);
+    fd = open(filename, O_RDWR | O_BINARY | O_LARGEFILE | O_SYNC);
     if (fd < 0) {
         fd = open(filename, O_RDONLY | O_BINARY | O_LARGEFILE);
         if (fd < 0)
--- xen-unstable-10712/tools/ioemu/block-cloop.c.osync  2006-07-28 
02:15:56.000000000 -0400
+++ xen-unstable-10712/tools/ioemu/block-cloop.c        2006-07-28 
02:17:13.000000000 -0400
@@ -55,7 +55,7 @@
     BDRVCloopState *s = bs->opaque;
     uint32_t offsets_size,max_compressed_block_size=1,i;
- s->fd = open(filename, O_RDONLY | O_BINARY | O_LARGEFILE);
+    s->fd = open(filename, O_RDONLY | O_BINARY | O_LARGEFILE | O_SYNC);
     if (s->fd < 0)
         return -1;
     bs->read_only = 1;
--- xen-unstable-10712/tools/ioemu/block-cow.c.osync    2006-07-28 
02:15:56.000000000 -0400
+++ xen-unstable-10712/tools/ioemu/block-cow.c  2006-07-28 02:21:34.000000000 
-0400
@@ -69,7 +69,7 @@
     struct cow_header_v2 cow_header;
     int64_t size;
- fd = open(filename, O_RDWR | O_BINARY | O_LARGEFILE);
+    fd = open(filename, O_RDWR | O_BINARY | O_LARGEFILE | O_SYNC);
     if (fd < 0) {
         fd = open(filename, O_RDONLY | O_BINARY | O_LARGEFILE);
         if (fd < 0)
--- xen-unstable-10712/tools/ioemu/block-qcow.c.osync   2006-07-28 
02:15:56.000000000 -0400
+++ xen-unstable-10712/tools/ioemu/block-qcow.c 2006-07-28 02:20:05.000000000 
-0400
@@ -95,7 +95,7 @@
     int fd, len, i, shift;
     QCowHeader header;
- fd = open(filename, O_RDWR | O_BINARY | O_LARGEFILE);
+    fd = open(filename, O_RDWR | O_BINARY | O_LARGEFILE | O_SYNC);
     if (fd < 0) {
         fd = open(filename, O_RDONLY | O_BINARY | O_LARGEFILE);
         if (fd < 0)
--- xen-unstable-10712/tools/ioemu/block-vmdk.c.osync   2006-07-28 
02:15:56.000000000 -0400
+++ xen-unstable-10712/tools/ioemu/block-vmdk.c 2006-07-28 02:20:20.000000000 
-0400
@@ -96,7 +96,7 @@
     uint32_t magic;
     int l1_size;
- fd = open(filename, O_RDWR | O_BINARY | O_LARGEFILE);
+    fd = open(filename, O_RDWR | O_BINARY | O_LARGEFILE | O_SYNC);
     if (fd < 0) {
         fd = open(filename, O_RDONLY | O_BINARY | O_LARGEFILE);
         if (fd < 0)


------------------------------------------------------------------------

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