# HG changeset patch
# User Andrew Warfield <andy@xxxxxxxxxxxxx>
# Node ID 3d36f020e62ad01c5784fabf652086710c591527
# Parent b219ea61a357385af107bc7261237f4bcfd35ae5
Update user manual to use blktap for file-based VBDs.
Signed-off-by: Andrew Warfield <andrew.warfield@xxxxxxxxxxxxx>
---
docs/src/user.tex | 64 ++++++++++++++++++++++++++++++++++++++++--------------
1 files changed, 48 insertions(+), 16 deletions(-)
diff -r b219ea61a357 -r 3d36f020e62a docs/src/user.tex
--- a/docs/src/user.tex Mon Sep 04 16:38:24 2006 +0100
+++ b/docs/src/user.tex Mon Sep 04 13:54:36 2006 -0700
@@ -1654,26 +1654,58 @@ Now unmount (this is important!):
In the configuration file set:
\begin{quote}
+ \verb_disk = ['tap:aio:/full/path/to/vm1disk,sda1,w']_
+\end{quote}
+
+As the virtual machine writes to its `disk', the sparse file will be
+filled in and consume more space up to the original 2GB.
+
+{\em{Note:}} Users that have worked with file-backed VBDs on Xen in previous
+versions will be interested to know that this support is not provided through
+the blktap driver instead of the loopback driver. This change results in
+file-based block devices that are higher-performance, more scalable, and which
+provide better safety properties for VBD data. All that is required to update
+your existing file-backed VM configurations is to change VBD configuration
+lines from:
+\begin{quote}
\verb_disk = ['file:/full/path/to/vm1disk,sda1,w']_
\end{quote}
-
-As the virtual machine writes to its `disk', the sparse file will be
-filled in and consume more space up to the original 2GB.
-
-{\bf Note that file-backed VBDs may not be appropriate for backing
- I/O-intensive domains.} File-backed VBDs are known to experience
+to:
+\begin{quote}
+ \verb_disk = ['tap:aio:/full/path/to/vm1disk,sda1,w']_
+\end{quote}
+
+
+\subsection{Loopback-mounted file-backed VBDs (deprecated)}
+
+{\em{{\bf{Note:}} Loopback mounted VBDs have now been replaced with
+ blktap-based support for raw image files, as described above. This
+ section remains to detail a configuration that was used by older Xen
+ versions.}}
+
+Raw image file-backed VBDs amy also be attached to VMs using the
+Linux loopback driver. The only required change to the raw file
+instructions above are to specify the configuration entry as:
+\begin{quote}
+ \verb_disk = ['file:/full/path/to/vm1disk,sda1,w']_
+\end{quote}
+
+{\bf Note that loopback file-backed VBDs may not be appropriate for backing
+ I/O-intensive domains.} This approach is known to experience
substantial slowdowns under heavy I/O workloads, due to the I/O
handling by the loopback block device used to support file-backed VBDs
-in dom0. Better I/O performance can be achieved by using either
-LVM-backed VBDs (Section~\ref{s:using-lvm-backed-vbds}) or physical
-devices as VBDs (Section~\ref{s:exporting-physical-devices-as-vbds}).
-
-Linux supports a maximum of eight file-backed VBDs across all domains
-by default. This limit can be statically increased by using the
-\emph{max\_loop} module parameter if CONFIG\_BLK\_DEV\_LOOP is
-compiled as a module in the dom0 kernel, or by using the
-\emph{max\_loop=n} boot option if CONFIG\_BLK\_DEV\_LOOP is compiled
-directly into the dom0 kernel.
+in dom0. Loopbach support remains for old Xen installations, and users
+are strongly encouraged to use the blktap-based file support (using
+``{\tt{tap:aio}}'' as described above).
+
+Additionally, Linux supports a maximum of eight loopback file-backed
+VBDs across all domains by default. This limit can be statically
+increased by using the \emph{max\_loop} module parameter if
+CONFIG\_BLK\_DEV\_LOOP is compiled as a module in the dom0 kernel, or
+by using the \emph{max\_loop=n} boot option if CONFIG\_BLK\_DEV\_LOOP
+is compiled directly into the dom0 kernel. Again, users are encouraged
+to use the blktap-based file support described above which scales to much
+larger number of active VBDs.
\section{Using LVM-backed VBDs}
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