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

[Fabrice Bellard <fabrice@bellard.org>]

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 <riel@redhat.com>
> 
> --- 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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