direct-io.hg

changeset 11414:3d36f020e62a

Update user manual to use blktap for file-based VBDs.

Signed-off-by: Andrew Warfield <andrew.warfield@xensource.com>
author Andrew Warfield <andy@xensource.com>
date Mon Sep 04 13:54:36 2006 -0700 (2006-09-04)
parents b219ea61a357
children 2d8d6ce64454 ba39c2da5e84
files docs/src/user.tex
line diff
     1.1 --- a/docs/src/user.tex	Mon Sep 04 16:38:24 2006 +0100
     1.2 +++ b/docs/src/user.tex	Mon Sep 04 13:54:36 2006 -0700
     1.3 @@ -1654,26 +1654,58 @@ Now unmount (this is important!):
     1.4  
     1.5  In the configuration file set:
     1.6  \begin{quote}
     1.7 -  \verb_disk = ['file:/full/path/to/vm1disk,sda1,w']_
     1.8 +  \verb_disk = ['tap:aio:/full/path/to/vm1disk,sda1,w']_
     1.9  \end{quote}
    1.10  
    1.11  As the virtual machine writes to its `disk', the sparse file will be
    1.12  filled in and consume more space up to the original 2GB.
    1.13  
    1.14 -{\bf Note that file-backed VBDs may not be appropriate for backing
    1.15 -  I/O-intensive domains.}  File-backed VBDs are known to experience
    1.16 +{\em{Note:}} Users that have worked with file-backed VBDs on Xen in previous
    1.17 +versions will be interested to know that this support is not provided through
    1.18 +the blktap driver instead of the loopback driver.  This change results in
    1.19 +file-based block devices that are higher-performance, more scalable, and which
    1.20 +provide better safety properties for VBD data.  All that is required to update
    1.21 +your existing file-backed VM configurations is to change VBD configuration
    1.22 +lines from:
    1.23 +\begin{quote}
    1.24 +  \verb_disk = ['file:/full/path/to/vm1disk,sda1,w']_
    1.25 +\end{quote}
    1.26 +to:
    1.27 +\begin{quote}
    1.28 +  \verb_disk = ['tap:aio:/full/path/to/vm1disk,sda1,w']_
    1.29 +\end{quote}
    1.30 +
    1.31 +
    1.32 +\subsection{Loopback-mounted file-backed VBDs (deprecated)}
    1.33 +
    1.34 +{\em{{\bf{Note:}} Loopback mounted VBDs have now been replaced with
    1.35 +    blktap-based support for raw image files, as described above.  This
    1.36 +    section remains to detail a configuration that was used by older Xen
    1.37 +    versions.}}
    1.38 +
    1.39 +Raw image file-backed VBDs amy also be attached to VMs using the 
    1.40 +Linux loopback driver.  The only required change to the raw file 
    1.41 +instructions above are to specify the configuration entry as:
    1.42 +\begin{quote}
    1.43 +  \verb_disk = ['file:/full/path/to/vm1disk,sda1,w']_
    1.44 +\end{quote}
    1.45 +
    1.46 +{\bf Note that loopback file-backed VBDs may not be appropriate for backing
    1.47 +  I/O-intensive domains.}  This approach is known to experience
    1.48  substantial slowdowns under heavy I/O workloads, due to the I/O
    1.49  handling by the loopback block device used to support file-backed VBDs
    1.50 -in dom0.  Better I/O performance can be achieved by using either
    1.51 -LVM-backed VBDs (Section~\ref{s:using-lvm-backed-vbds}) or physical
    1.52 -devices as VBDs (Section~\ref{s:exporting-physical-devices-as-vbds}).
    1.53 -
    1.54 -Linux supports a maximum of eight file-backed VBDs across all domains
    1.55 -by default.  This limit can be statically increased by using the
    1.56 -\emph{max\_loop} module parameter if CONFIG\_BLK\_DEV\_LOOP is
    1.57 -compiled as a module in the dom0 kernel, or by using the
    1.58 -\emph{max\_loop=n} boot option if CONFIG\_BLK\_DEV\_LOOP is compiled
    1.59 -directly into the dom0 kernel.
    1.60 +in dom0.  Loopbach support remains for old Xen installations, and users
    1.61 +are strongly encouraged to use the blktap-based file support (using 
    1.62 +``{\tt{tap:aio}}'' as described above).
    1.63 +
    1.64 +Additionally, Linux supports a maximum of eight loopback file-backed 
    1.65 +VBDs across all domains by default.  This limit can be statically 
    1.66 +increased by using the \emph{max\_loop} module parameter if 
    1.67 +CONFIG\_BLK\_DEV\_LOOP is compiled as a module in the dom0 kernel, or 
    1.68 +by using the \emph{max\_loop=n} boot option if CONFIG\_BLK\_DEV\_LOOP 
    1.69 +is compiled directly into the dom0 kernel.  Again, users are encouraged
    1.70 +to use the blktap-based file support described above which scales to much 
    1.71 +larger number of active VBDs.
    1.72  
    1.73  
    1.74  \section{Using LVM-backed VBDs}