mirror of
https://github.com/libguestfs/libguestfs.git
synced 2026-03-22 07:03:38 +00:00
2910413850
The first partition can now be aligned. We fix the bootloader correctly for Windows by adjusting the "Hidden Sectors" field.
367 lines
10 KiB
Plaintext
Executable File
367 lines
10 KiB
Plaintext
Executable File
=encoding utf8
|
|
|
|
=head1 NAME
|
|
|
|
virt-alignment-scan - Check alignment of virtual machine partitions
|
|
|
|
=head1 SYNOPSIS
|
|
|
|
virt-alignment-scan [--options] -d domname
|
|
|
|
virt-alignment-scan [--options] -a disk.img [-a disk.img ...]
|
|
|
|
=head1 DESCRIPTION
|
|
|
|
When older operating systems install themselves, the partitioning
|
|
tools place partitions at a sector misaligned with the underlying
|
|
storage (commonly the first partition starts on sector C<63>).
|
|
Misaligned partitions can result in an operating system issuing more
|
|
I/O than should be necessary.
|
|
|
|
The virt-alignment-scan tool checks the alignment of partitions in
|
|
virtual machines and disk images and warns you if there are alignment
|
|
problems.
|
|
|
|
Currently there is no virt tool for fixing alignment problems. You
|
|
can only reinstall the guest operating system. The following NetApp
|
|
document summarises the problem and possible solutions:
|
|
L<http://media.netapp.com/documents/tr-3747.pdf>
|
|
|
|
=head1 OUTPUT
|
|
|
|
To run this tool on a disk image directly, use the I<-a> option:
|
|
|
|
$ virt-alignment-scan -a winxp.img
|
|
/dev/sda1 32256 512 bad (alignment < 4K)
|
|
|
|
$ virt-alignment-scan -a fedora16.img
|
|
/dev/sda1 1048576 1024K ok
|
|
/dev/sda2 2097152 2048K ok
|
|
/dev/sda3 526385152 2048K ok
|
|
|
|
To run the tool on a guest known to libvirt, use the I<-d> option and
|
|
possibly the I<-c> option:
|
|
|
|
# virt-alignment-scan -d RHEL5
|
|
/dev/sda1 32256 512 bad (alignment < 4K)
|
|
/dev/sda2 106928640 512 bad (alignment < 4K)
|
|
|
|
$ virt-alignment-scan -c qemu:///system -d Win7TwoDisks
|
|
/dev/sda1 1048576 1024K ok
|
|
/dev/sda2 105906176 1024K ok
|
|
/dev/sdb1 65536 64K ok
|
|
|
|
The output consists of 4 or more whitespace-separated columns. Only
|
|
the first 4 columns are signficant if you want to parse this from a
|
|
program. The columns are:
|
|
|
|
=over 4
|
|
|
|
=item col 1
|
|
|
|
the device and partition name (eg. C</dev/sda1> meaning the
|
|
first partition on the first block device)
|
|
|
|
=item col 2
|
|
|
|
the start of the partition in bytes
|
|
|
|
=item col 3
|
|
|
|
the alignment in bytes or Kbytes (eg. C<512> or C<4K>)
|
|
|
|
=item col 4
|
|
|
|
C<ok> if the alignment is best for performance, or C<bad> if the
|
|
alignment can cause performance problems
|
|
|
|
=item cols 5+
|
|
|
|
optional free-text explanation.
|
|
|
|
=back
|
|
|
|
The exit code from the program changes depending on whether poorly
|
|
aligned partitions were found. See L</EXIT STATUS> below.
|
|
|
|
If you just want the exit code with no output, use the I<-q> option.
|
|
|
|
=head1 OPTIONS
|
|
|
|
=over 4
|
|
|
|
=item B<--help>
|
|
|
|
Display brief help.
|
|
|
|
=item B<-a> file
|
|
|
|
=item B<--add> file
|
|
|
|
Add I<file> which should be a disk image from a virtual machine.
|
|
|
|
The format of the disk image is auto-detected. To override this and
|
|
force a particular format use the I<--format=..> option.
|
|
|
|
=item B<-c> URI
|
|
|
|
=item B<--connect> URI
|
|
|
|
If using libvirt, connect to the given I<URI>. If omitted, then we
|
|
connect to the default libvirt hypervisor.
|
|
|
|
If you specify guest block devices directly (I<-a>), then libvirt is
|
|
not used at all.
|
|
|
|
=item B<-d> guest
|
|
|
|
=item B<--domain> guest
|
|
|
|
Add all the disks from the named libvirt guest. Domain UUIDs can be
|
|
used instead of names.
|
|
|
|
=item B<--format=raw|qcow2|..>
|
|
|
|
=item B<--format>
|
|
|
|
The default for the I<-a> option is to auto-detect the format of the
|
|
disk image. Using this forces the disk format for I<-a> options which
|
|
follow on the command line. Using I<--format> with no argument
|
|
switches back to auto-detection for subsequent I<-a> options.
|
|
|
|
For example:
|
|
|
|
virt-alignment-scan --format=raw -a disk.img
|
|
|
|
forces raw format (no auto-detection) for C<disk.img>.
|
|
|
|
virt-alignment-scan --format=raw -a disk.img --format -a another.img
|
|
|
|
forces raw format (no auto-detection) for C<disk.img> and reverts to
|
|
auto-detection for C<another.img>.
|
|
|
|
If you have untrusted raw-format guest disk images, you should use
|
|
this option to specify the disk format. This avoids a possible
|
|
security problem with malicious guests (CVE-2010-3851).
|
|
|
|
=item B<-q>
|
|
|
|
=item B<--quiet>
|
|
|
|
Don't produce any output. Just set the exit code
|
|
(see L</EXIT STATUS> below).
|
|
|
|
=item B<-v>
|
|
|
|
=item B<--verbose>
|
|
|
|
Enable verbose messages for debugging.
|
|
|
|
=item B<-V>
|
|
|
|
=item B<--version>
|
|
|
|
Display version number and exit.
|
|
|
|
=item B<-x>
|
|
|
|
Enable tracing of libguestfs API calls.
|
|
|
|
=back
|
|
|
|
=head1 RECOMMENDED ALIGNMENT
|
|
|
|
Operating systems older than Windows 2008 and Linux before ca.2010
|
|
place the first sector of the first partition at sector 63, with a 512
|
|
byte sector size. This happens because of a historical accident.
|
|
Drives have to report a cylinder / head / sector (CHS) geometry to the
|
|
BIOS. The geometry is completely meaningless on modern drives, but it
|
|
happens that the geometry reported always has 63 sectors per track.
|
|
The operating system therefore places the first partition at the start
|
|
of the second "track", at sector 63.
|
|
|
|
When the guest OS is virtualized, the host operating system and
|
|
hypervisor may prefer accesses aligned to one of:
|
|
|
|
=over 4
|
|
|
|
=item * 512 bytes
|
|
|
|
if the host OS uses local storage directly on hard drive partitions,
|
|
and the hard drive has 512 byte physical sectors.
|
|
|
|
=item * 4 Kbytes
|
|
|
|
for local storage on new hard drives with 4Kbyte physical sectors; for
|
|
file-backed storage on filesystems with 4Kbyte block size; or for some
|
|
types of network-attached storage.
|
|
|
|
=item * 64 Kbytes
|
|
|
|
for high-end network-attached storage. This is the optimal block size
|
|
for some NetApp hardware.
|
|
|
|
=item * 1 Mbyte
|
|
|
|
see L</1 MB PARTITION ALIGNMENT> below.
|
|
|
|
=back
|
|
|
|
Partitions which are not aligned correctly to the underlying
|
|
storage cause extra I/O. For example:
|
|
|
|
sect#63
|
|
+--------------------------+------
|
|
| guest |
|
|
| filesystem block |
|
|
---+------------------+------+-------------------+-----+---
|
|
| host block | host block |
|
|
| | |
|
|
---+-------------------------+-------------------------+---
|
|
|
|
In this example, each time a 4K guest block is read, two blocks on the
|
|
host must be accessed (so twice as much I/O is done). When a 4K guest
|
|
block is written, two host blocks must first be read, the old and new
|
|
data combined, and the two blocks written back (4x I/O).
|
|
|
|
=head2 LINUX HOST BLOCK AND I/O SIZE
|
|
|
|
New versions of the Linux kernel expose the physical and logical block
|
|
size, and minimum and recommended I/O size.
|
|
|
|
For a typical hard drive with 512 byte sectors:
|
|
|
|
$ cat /sys/block/sda/queue/physical_block_size
|
|
512
|
|
$ cat /sys/block/sda/queue/logical_block_size
|
|
512
|
|
$ cat /sys/block/sda/queue/minimum_io_size
|
|
512
|
|
$ cat /sys/block/sda/queue/optimal_io_size
|
|
0
|
|
|
|
For a NetApp LUN:
|
|
|
|
$ cat /sys/block/sdc/queue/logical_block_size
|
|
512
|
|
$ cat /sys/block/sdc/queue/physical_block_size
|
|
512
|
|
$ cat /sys/block/sdc/queue/minimum_io_size
|
|
4096
|
|
$ cat /sys/block/sdc/queue/optimal_io_size
|
|
65536
|
|
|
|
The NetApp allows 512 byte accesses (but they will be very
|
|
inefficient), prefers a minimum 4K I/O size, but the optimal I/O size
|
|
is 64K.
|
|
|
|
For detailed information about what these numbers mean, see
|
|
L<http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Storage_Administration_Guide/newstorage-iolimits.html>
|
|
|
|
[Thanks to Mike Snitzer for providing NetApp data and additional
|
|
information.]
|
|
|
|
=head2 1 MB PARTITION ALIGNMENT
|
|
|
|
Microsoft picked 1 MB as the default alignment for all partitions
|
|
starting with Windows 2008 Server, and Linux has followed this.
|
|
|
|
Assuming 512 byte sectors in the guest, you will now see the first
|
|
partition starting at sector 2048, and subsequent partitions (if any)
|
|
will start at a multiple of 2048 sectors.
|
|
|
|
1 MB alignment is compatible with all current alignment requirements
|
|
(4K, 64K) and provides room for future growth in physical block sizes.
|
|
|
|
=head2 SETTING ALIGNMENT
|
|
|
|
L<virt-resize(1)> can change the alignment of the partitions of some
|
|
guests. Currently it can fully align all the partitions of all
|
|
Windows guests, and it will fix the bootloader where necessary. For
|
|
Linux guests, it can align the second and subsequent partitions, so
|
|
the majority of OS accesses except at boot will be aligned.
|
|
|
|
Another way to correct partition alignment problems is to reinstall
|
|
your guest operating systems. If you install operating systems from
|
|
templates, ensure these have correct partition alignment too.
|
|
|
|
For older versions of Windows, the following NetApp document contains
|
|
useful information: L<http://media.netapp.com/documents/tr-3747.pdf>
|
|
|
|
For Red Hat Enterprise Linux E<le> 5, use a Kickstart script that
|
|
contains an explicit C<%pre> section that creates aligned partitions
|
|
using L<parted(8)>. Do not use the Kickstart C<part> command. The
|
|
NetApp document above contains an example.
|
|
|
|
=head1 SHELL QUOTING
|
|
|
|
Libvirt guest names can contain arbitrary characters, some of which
|
|
have meaning to the shell such as C<#> and space. You may need to
|
|
quote or escape these characters on the command line. See the shell
|
|
manual page L<sh(1)> for details.
|
|
|
|
=head1 EXIT STATUS
|
|
|
|
This program returns:
|
|
|
|
=over 4
|
|
|
|
=item *
|
|
|
|
0
|
|
|
|
successful exit, all partitions are aligned E<ge> 64K for best performance
|
|
|
|
=item *
|
|
|
|
1
|
|
|
|
an error scanning the disk image or guest
|
|
|
|
=item *
|
|
|
|
2
|
|
|
|
successful exit, some partitions have alignment E<lt> 64K which can result
|
|
in poor performance on high end network storage
|
|
|
|
=item *
|
|
|
|
3
|
|
|
|
successful exit, some partitions have alignment E<lt> 4K which can result
|
|
in poor performance on most hypervisors
|
|
|
|
=back
|
|
|
|
=head1 SEE ALSO
|
|
|
|
L<guestfs(3)>,
|
|
L<guestfish(1)>,
|
|
L<virt-filesystems(1)>,
|
|
L<virt-rescue(1)>,
|
|
L<virt-resize(1)>,
|
|
L<http://libguestfs.org/>.
|
|
|
|
=head1 AUTHOR
|
|
|
|
Richard W.M. Jones L<http://people.redhat.com/~rjones/>
|
|
|
|
=head1 COPYRIGHT
|
|
|
|
Copyright (C) 2011 Red Hat Inc.
|
|
|
|
This program is free software; you can redistribute it and/or modify
|
|
it under the terms of the GNU General Public License as published by
|
|
the Free Software Foundation; either version 2 of the License, or
|
|
(at your option) any later version.
|
|
|
|
This program is distributed in the hope that it will be useful,
|
|
but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
|
|
GNU General Public License for more details.
|
|
|
|
You should have received a copy of the GNU General Public License
|
|
along with this program; if not, write to the Free Software
|
|
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
|