From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:54918) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QotaI-0004DY-Jn for qemu-devel@nongnu.org; Thu, 04 Aug 2011 04:47:19 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QotaG-0002Xu-9a for qemu-devel@nongnu.org; Thu, 04 Aug 2011 04:47:18 -0400 Received: from mtagate7.uk.ibm.com ([194.196.100.167]:49070) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QotaG-0002UK-2I for qemu-devel@nongnu.org; Thu, 04 Aug 2011 04:47:16 -0400 Received: from d06nrmr1806.portsmouth.uk.ibm.com (d06nrmr1806.portsmouth.uk.ibm.com [9.149.39.193]) by mtagate7.uk.ibm.com (8.13.1/8.13.1) with ESMTP id p748kqL3025125 for ; Thu, 4 Aug 2011 08:46:52 GMT Received: from d06av10.portsmouth.uk.ibm.com (d06av10.portsmouth.uk.ibm.com [9.149.37.251]) by d06nrmr1806.portsmouth.uk.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id p748kqTF2683126 for ; Thu, 4 Aug 2011 09:46:52 +0100 Received: from d06av10.portsmouth.uk.ibm.com (loopback [127.0.0.1]) by d06av10.portsmouth.uk.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id p748kqff026268 for ; Thu, 4 Aug 2011 02:46:52 -0600 From: Stefan Hajnoczi Date: Thu, 4 Aug 2011 09:46:34 +0100 Message-Id: <1312447594-24046-2-git-send-email-stefanha@linux.vnet.ibm.com> In-Reply-To: <1312447594-24046-1-git-send-email-stefanha@linux.vnet.ibm.com> References: <1312447594-24046-1-git-send-email-stefanha@linux.vnet.ibm.com> Subject: [Qemu-devel] [PATCH v2 2/2] block: add cache=directsync parameter to -drive List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: Kevin Wolf , Stefan Hajnoczi This patch adds -drive cache=directsync for O_DIRECT | O_SYNC host file I/O with no disk write cache presented to the guest. This mode is useful when guests may not be sending flushes when appropriate and therefore leave data at risk in case of power failure. When cache=directsync is used, write operations are only completed to the guest when data is safely on disk. This new mode is like cache=writethrough but it bypasses the host page cache. Signed-off-by: Stefan Hajnoczi --- block.c | 6 ++++-- qemu-config.c | 3 ++- qemu-img.c | 3 ++- qemu-options.hx | 8 ++++++-- 4 files changed, 14 insertions(+), 6 deletions(-) diff --git a/block.c b/block.c index 4aba434..bce6b9d 100644 --- a/block.c +++ b/block.c @@ -423,6 +423,8 @@ int bdrv_set_cache_flags(const char *mode, int *flags) if (!strcmp(mode, "off") || !strcmp(mode, "none")) { *flags |= BDRV_O_NOCACHE | BDRV_O_CACHE_WB; + } else if (!strcmp(mode, "directsync")) { + *flags |= BDRV_O_NOCACHE; } else if (!strcmp(mode, "writeback")) { *flags |= BDRV_O_CACHE_WB; } else if (!strcmp(mode, "unsafe")) { @@ -1125,8 +1127,8 @@ int bdrv_pwrite_sync(BlockDriverState *bs, int64_t offset, return ret; } - /* No flush needed for cache=writethrough, it uses O_DSYNC */ - if ((bs->open_flags & BDRV_O_CACHE_MASK) != 0) { + /* No flush needed for cache modes that use O_DSYNC */ + if ((bs->open_flags & BDRV_O_CACHE_WB) != 0) { bdrv_flush(bs); } diff --git a/qemu-config.c b/qemu-config.c index 1eb6b9a..139e077 100644 --- a/qemu-config.c +++ b/qemu-config.c @@ -55,7 +55,8 @@ static QemuOptsList qemu_drive_opts = { },{ .name = "cache", .type = QEMU_OPT_STRING, - .help = "host cache usage (none, writeback, writethrough, unsafe)", + .help = "host cache usage (none, writeback, writethrough, " + "directsync, unsafe)", },{ .name = "aio", .type = QEMU_OPT_STRING, diff --git a/qemu-img.c b/qemu-img.c index f39c8c5..068a600 100644 --- a/qemu-img.c +++ b/qemu-img.c @@ -66,7 +66,8 @@ static void help(void) " 'filename' is a disk image filename\n" " 'fmt' is the disk image format. It is guessed automatically in most cases\n" " 'cache' is the cache mode used to write the output disk image, the valid\n" - " options are: 'none', 'writeback' (default), 'writethrough' and 'unsafe'\n" + " options are: 'none', 'writeback' (default), 'writethrough', 'directsync'\n" + " and 'unsafe'\n" " 'size' is the disk image size in bytes. Optional suffixes\n" " 'k' or 'K' (kilobyte, 1024), 'M' (megabyte, 1024k), 'G' (gigabyte, 1024M)\n" " and T (terabyte, 1024G) are supported. 'b' is ignored.\n" diff --git a/qemu-options.hx b/qemu-options.hx index d86815d..35d95d1 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -133,7 +133,7 @@ ETEXI DEF("drive", HAS_ARG, QEMU_OPTION_drive, "-drive [file=file][,if=type][,bus=n][,unit=m][,media=d][,index=i]\n" " [,cyls=c,heads=h,secs=s[,trans=t]][,snapshot=on|off]\n" - " [,cache=writethrough|writeback|none|unsafe][,format=f]\n" + " [,cache=writethrough|writeback|none|directsync|unsafe][,format=f]\n" " [,serial=s][,addr=A][,id=name][,aio=threads|native]\n" " [,readonly=on|off]\n" " use 'file' as a drive image\n", QEMU_ARCH_ALL) @@ -164,7 +164,7 @@ These options have the same definition as they have in @option{-hdachs}. @item snapshot=@var{snapshot} @var{snapshot} is "on" or "off" and allows to enable snapshot for given drive (see @option{-snapshot}). @item cache=@var{cache} -@var{cache} is "none", "writeback", "unsafe", or "writethrough" and controls how the host cache is used to access block data. +@var{cache} is "none", "writeback", "unsafe", "directsync" or "writethrough" and controls how the host cache is used to access block data. @item aio=@var{aio} @var{aio} is "threads", or "native" and selects between pthread based disk I/O and native Linux AIO. @item format=@var{format} @@ -199,6 +199,10 @@ The host page cache can be avoided entirely with @option{cache=none}. This will attempt to do disk IO directly to the guests memory. QEMU may still perform an internal copy of the data. +The host page cache can be avoided while only sending write notifications to +the guest when the data has been reported as written by the storage subsystem +using @option{cache=directsync}. + Some block drivers perform badly with @option{cache=writethrough}, most notably, qcow2. If performance is more important than correctness, @option{cache=writeback} should be used with qcow2. -- 1.7.5.4