From ff0d6d8c347f620091c3477703d6f827fddf7216 Mon Sep 17 00:00:00 2001 Message-Id: In-Reply-To: <54163d537ed24926effb0783707492d2988ecbe8.1424280081.git.jen@redhat.com> References: <54163d537ed24926effb0783707492d2988ecbe8.1424280081.git.jen@redhat.com> From: Stefan Hajnoczi Date: Thu, 5 Feb 2015 10:11:42 -0500 Subject: [CHANGE 2/8] block: add cache=directsync parameter to -drive To: rhvirt-patches@redhat.com, jen@redhat.com RH-Author: Stefan Hajnoczi Message-id: <1423131102-24943-1-git-send-email-stefanha@redhat.com> Patchwork-id: 63726 O-Subject: [RHEL-6.7 qemu-kvm PATCH] block: add cache=directsync parameter to -drive Bugzilla: 1186914 RH-Acked-by: Kevin Wolf RH-Acked-by: Fam Zheng RH-Acked-by: Paolo Bonzini From: Stefan Hajnoczi Bugzilla: 1186914 Brew: http://brewweb.devel.redhat.com/brew/taskinfo?taskID=8693223 Upstream: merged 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 Signed-off-by: Kevin Wolf (cherry picked from commit 92196b2f5664d5defa778b1b24df56e2239b5e93) Signed-off-by: Stefan Hajnoczi Signed-off-by: Jeff E. Nelson Conflicts: qemu-img.c The help text in qemu-img.c has a context conflict. Easy to resolve. --- block.c | 6 ++++-- qemu-config.c | 3 ++- qemu-img.c | 4 ++-- qemu-options.hx | 8 ++++++-- 4 files changed, 14 insertions(+), 7 deletions(-) Signed-off-by: Jeff E. Nelson --- block.c | 6 ++++-- qemu-config.c | 3 ++- qemu-img.c | 4 ++-- qemu-options.hx | 8 ++++++-- 4 files changed, 14 insertions(+), 7 deletions(-) diff --git a/block.c b/block.c index b562da7..29bab36 100644 --- a/block.c +++ b/block.c @@ -551,6 +551,8 @@ int bdrv_parse_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")) { @@ -2105,8 +2107,8 @@ 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 411b22f..cca8952 100644 --- a/qemu-config.c +++ b/qemu-config.c @@ -54,7 +54,8 @@ 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 bc31cf2..8252173 100644 --- a/qemu-img.c +++ b/qemu-img.c @@ -78,8 +78,8 @@ 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" - " 'src_cache' in contrast is the cache mode used to read input disk images\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 a23da0b..8fdf2eb 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -102,7 +102,7 @@ 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][,copy-on-read=on|off]\n" " [[,bps=b]|[[,bps_rd=r][,bps_wr=w]]][[,iops=i]|[[,iops_rd=r][,iops_wr=w]]\n" @@ -133,7 +133,7 @@ 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} @@ -172,6 +172,10 @@ 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. -- 2.1.0