[Qemu-devel] [PATCH 1/2] Documentation: Update block cache mode information

[Kevin Wolf <kwolf@redhat.com>]

Somehow we forgot to update this when cache=writeback became the
default. While changing the information on the default, also make the
description of all caches modes a bit more accurate.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 qemu-options.hx |   34 +++++++++++++++++-----------------
 1 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/qemu-options.hx b/qemu-options.hx
index 9bb29d3..cb59e3e 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -206,31 +206,31 @@ Open drive @option{file} as read-only. Guest write attempts will fail.
 file sectors into the image file.
 @end table
 
-By default, writethrough caching is used for all block device.  This means that
-the host page cache will be used to read and write data but write notification
-will be sent to the guest only when the data has been reported as written by
-the storage subsystem.
-
-Writeback caching will report data writes as completed as soon as the data is
-present in the host page cache.  This is safe as long as you trust your host.
-If your host crashes or loses power, then the guest may experience data
-corruption.
+By default, the @option{cache=writeback} mode is used. It will report data
+writes as completed as soon as the data is present in the host page cache.
+This is safe as long as your guest OS makes sure to correctly flush disk caches
+where needed. If your guest OS does not handle volatile disk write caches
+correctly and your host crashes or loses power, then the guest may experience
+data corruption.
+
+For such guests, you should consider using @option{cache=writethrough}. This
+means that the host page cache will be used to read and write data, but write
+notification will be sent to the guest only after qemu has made sure to flush
+each write to the disk. Be aware that this has a major impact on performance.
 
 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.
+an internal copy of the data. Note that this is considered a writeback mode and
+the guest OS must handle the disk write cache correctly in order to avoid data
+corruption on host crashes.
 
 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.
+the guest when the data has been flushed to the disk using
+@option{cache=directsync}.
 
 In case you don't care about data integrity over host failures, use
-cache=unsafe. This option tells QEMU that it never needs to write any data
-to the disk but can instead keeps things in cache. If anything goes wrong,
+@option{cache=unsafe}. This option tells QEMU that it never needs to write any
+data to the disk but can instead keeps things in cache. If anything goes wrong,
 like your host losing power, the disk storage getting disconnected accidentally,
 etc. you're image will most probably be rendered unusable.   When using
 the @option{-snapshot} option, unsafe caching is always used.
-- 
1.7.6.5