5 virt-alignment-scan - Check alignment of virtual machine partitions
9 virt-alignment-scan [--options]
11 virt-alignment-scan [--options] -d domname
13 virt-alignment-scan [--options] -a disk.img [-a disk.img ...]
17 When older operating systems install themselves, the partitioning
18 tools place partitions at a sector misaligned with the underlying
19 storage (commonly the first partition starts on sector C<63>).
20 Misaligned partitions can result in an operating system issuing more
21 I/O than should be necessary.
23 The virt-alignment-scan tool checks the alignment of partitions in
24 virtual machines and disk images and warns you if there are alignment
27 Currently there is no virt tool for fixing alignment problems. You
28 can only reinstall the guest operating system. The following NetApp
29 document summarises the problem and possible solutions:
30 L<http://media.netapp.com/documents/tr-3747.pdf>
34 To run this tool on a disk image directly, use the I<-a> option:
36 $ virt-alignment-scan -a winxp.img
37 /dev/sda1 32256 512 bad (alignment < 4K)
39 $ virt-alignment-scan -a fedora16.img
40 /dev/sda1 1048576 1024K ok
41 /dev/sda2 2097152 2048K ok
42 /dev/sda3 526385152 2048K ok
44 To run the tool on a guest known to libvirt, use the I<-d> option and
45 possibly the I<-c> option:
47 # virt-alignment-scan -d RHEL5
48 /dev/sda1 32256 512 bad (alignment < 4K)
49 /dev/sda2 106928640 512 bad (alignment < 4K)
51 $ virt-alignment-scan -c qemu:///system -d Win7TwoDisks
52 /dev/sda1 1048576 1024K ok
53 /dev/sda2 105906176 1024K ok
54 /dev/sdb1 65536 64K ok
56 The output consists of 4 or more whitespace-separated columns. Only
57 the first 4 columns are signficant if you want to parse this from a
58 program. The columns are:
64 the device and partition name (eg. C</dev/sda1> meaning the
65 first partition on the first block device)
69 the start of the partition in bytes
73 the alignment in bytes or Kbytes (eg. C<512> or C<4K>)
77 C<ok> if the alignment is best for performance, or C<bad> if the
78 alignment can cause performance problems
82 optional free-text explanation.
86 The exit code from the program changes depending on whether poorly
87 aligned partitions were found. See L</EXIT STATUS> below.
89 If you just want the exit code with no output, use the I<-q> option.
103 Add I<file> which should be a disk image from a virtual machine.
105 The format of the disk image is auto-detected. To override this and
106 force a particular format use the I<--format=..> option.
110 =item B<--connect> URI
112 If using libvirt, connect to the given I<URI>. If omitted, then we
113 connect to the default libvirt hypervisor.
115 If you specify guest block devices directly (I<-a>), then libvirt is
120 =item B<--domain> guest
122 Add all the disks from the named libvirt guest. Domain UUIDs can be
123 used instead of names.
125 =item B<--format=raw|qcow2|..>
129 The default for the I<-a> option is to auto-detect the format of the
130 disk image. Using this forces the disk format for I<-a> options which
131 follow on the command line. Using I<--format> with no argument
132 switches back to auto-detection for subsequent I<-a> options.
136 virt-alignment-scan --format=raw -a disk.img
138 forces raw format (no auto-detection) for C<disk.img>.
140 virt-alignment-scan --format=raw -a disk.img --format -a another.img
142 forces raw format (no auto-detection) for C<disk.img> and reverts to
143 auto-detection for C<another.img>.
145 If you have untrusted raw-format guest disk images, you should use
146 this option to specify the disk format. This avoids a possible
147 security problem with malicious guests (CVE-2010-3851).
153 Don't produce any output. Just set the exit code
154 (see L</EXIT STATUS> below).
160 Enable verbose messages for debugging.
166 Display version number and exit.
170 Enable tracing of libguestfs API calls.
174 =head1 RECOMMENDED ALIGNMENT
176 Operating systems older than Windows 2008 and Linux before ca.2010
177 place the first sector of the first partition at sector 63, with a 512
178 byte sector size. This happens because of a historical accident.
179 Drives have to report a cylinder / head / sector (CHS) geometry to the
180 BIOS. The geometry is completely meaningless on modern drives, but it
181 happens that the geometry reported always has 63 sectors per track.
182 The operating system therefore places the first partition at the start
183 of the second "track", at sector 63.
185 When the guest OS is virtualized, the host operating system and
186 hypervisor may prefer accesses aligned to one of:
192 if the host OS uses local storage directly on hard drive partitions,
193 and the hard drive has 512 byte physical sectors.
197 for local storage on new hard drives with 4Kbyte physical sectors; for
198 file-backed storage on filesystems with 4Kbyte block size; or for some
199 types of network-attached storage.
203 for high-end network-attached storage. This is the optimal block size
204 for some NetApp hardware.
208 see L</1 MB PARTITION ALIGNMENT> below.
212 Partitions which are not aligned correctly to the underlying
213 storage cause extra I/O. For example:
216 +--------------------------+------
219 ---+------------------+------+-------------------+-----+---
220 | host block | host block |
222 ---+-------------------------+-------------------------+---
224 In this example, each time a 4K guest block is read, two blocks on the
225 host must be accessed (so twice as much I/O is done). When a 4K guest
226 block is written, two host blocks must first be read, the old and new
227 data combined, and the two blocks written back (4x I/O).
229 =head2 LINUX HOST BLOCK AND I/O SIZE
231 New versions of the Linux kernel expose the physical and logical block
232 size, and minimum and recommended I/O size.
234 For a typical hard drive with 512 byte sectors:
236 $ cat /sys/block/sda/queue/physical_block_size
238 $ cat /sys/block/sda/queue/logical_block_size
240 $ cat /sys/block/sda/queue/minimum_io_size
242 $ cat /sys/block/sda/queue/optimal_io_size
247 $ cat /sys/block/sdc/queue/logical_block_size
249 $ cat /sys/block/sdc/queue/physical_block_size
251 $ cat /sys/block/sdc/queue/minimum_io_size
253 $ cat /sys/block/sdc/queue/optimal_io_size
256 The NetApp allows 512 byte accesses (but they will be very
257 inefficient), prefers a minimum 4K I/O size, but the optimal I/O size
260 [Thanks to Mike Snitzer for providing NetApp data and additional
263 =head2 1 MB PARTITION ALIGNMENT
265 Microsoft picked 1 MB as the default alignment for all partitions
266 starting with Windows 2008 Server, and Linux has followed this.
268 Assuming 512 byte sectors in the guest, you will now see the first
269 partition starting at sector 2048, and subsequent partitions (if any)
270 will start at a multiple of 2048 sectors.
272 1 MB alignment is compatible with all current alignment requirements
273 (4K, 64K) and provides room for future growth in physical block sizes.
275 =head2 SETTING ALIGNMENT
277 Currently there is no virt tool for fixing alignment problems in
278 guests. This is a difficult problem to fix because simply moving
279 partitions around breaks the bootloader, necessitating either manual
280 reinstallation of the bootloader using a rescue disk, or complex and
283 L<virt-resize(1)> does not change the alignment of the first
284 partition, but it does align the second and subsequent partitions to a
285 multiple of 64 or 128 sectors (depending on the version of
286 virt-resize, 128 in virt-resize E<ge> 1.13.19). For operating systems
287 that have a separate boot partition, virt-resize could be used to
288 align the main OS partition, so that the majority of OS accesses
289 except at boot will be aligned.
291 The easiest way to correct partition alignment problems is to
292 reinstall your guest operating systems. If you install operating
293 systems from templates, ensure these have correct partition alignment
296 For older versions of Windows, the following NetApp document contains
297 useful information: L<http://media.netapp.com/documents/tr-3747.pdf>
299 For Red Hat Enterprise Linux E<le> 5, use a Kickstart script that
300 contains an explicit C<%pre> section that creates aligned partitions
301 using L<parted(8)>. Do not use the Kickstart C<part> command. The
302 NetApp document above contains an example.
306 Libvirt guest names can contain arbitrary characters, some of which
307 have meaning to the shell such as C<#> and space. You may need to
308 quote or escape these characters on the command line. See the shell
309 manual page L<sh(1)> for details.
313 This program returns:
321 successful exit, all partitions are aligned E<ge> 64K for best performance
327 an error scanning the disk image or guest
333 successful exit, some partitions have alignment E<lt> 64K which can result
334 in poor performance on high end network storage
340 successful exit, some partitions have alignment E<lt> 4K which can result
341 in poor performance on most hypervisors
349 L<virt-filesystems(1)>,
352 L<http://libguestfs.org/>.
356 Richard W.M. Jones L<http://people.redhat.com/~rjones/>
360 Copyright (C) 2011 Red Hat Inc.
362 This program is free software; you can redistribute it and/or modify
363 it under the terms of the GNU General Public License as published by
364 the Free Software Foundation; either version 2 of the License, or
365 (at your option) any later version.
367 This program is distributed in the hope that it will be useful,
368 but WITHOUT ANY WARRANTY; without even the implied warranty of
369 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
370 GNU General Public License for more details.
372 You should have received a copy of the GNU General Public License
373 along with this program; if not, write to the Free Software
374 Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.