1virt-alignment-scan(1) Virtualization Support virt-alignment-scan(1)
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6 virt-alignment-scan - Check alignment of virtual machine partitions
7
9 virt-alignment-scan [--options] -d domname
10 virt-alignment-scan [--options] -a disk.img [-a disk.img ...]
12 virt-alignment-scan [--options]
14
16 When older operating systems install themselves, the partitioning tools
17 place partitions at a sector misaligned with the underlying storage
18 (commonly the first partition starts on sector 63). Misaligned
19 partitions can result in an operating system issuing more I/O than
20 should be necessary.
21 The virt-alignment-scan tool checks the alignment of partitions in
23 virtual machines and disk images and warns you if there are alignment
24 problems.
25 Currently there is no virt tool for fixing alignment problems. You can
27 only reinstall the guest operating system. The following NetApp
28 document summarises the problem and possible solutions:
29 http://media.netapp.com/documents/tr-3747.pdf
30
32 To run this tool on a disk image directly, use the -a option:
33 $ virt-alignment-scan -a winxp.img
35 /dev/sda1 32256 512 bad (alignment < 4K)
36 $ virt-alignment-scan -a fedora16.img
38 /dev/sda1 1048576 1024K ok
39 /dev/sda2 2097152 2048K ok
40 /dev/sda3 526385152 2048K ok
41 To run the tool on a guest known to libvirt, use the -d option and
43 possibly the -c option:
44 # virt-alignment-scan -d RHEL5
46 /dev/sda1 32256 512 bad (alignment < 4K)
47 /dev/sda2 106928640 512 bad (alignment < 4K)
48 $ virt-alignment-scan -c qemu:///system -d Win7TwoDisks
50 /dev/sda1 1048576 1024K ok
51 /dev/sda2 105906176 1024K ok
52 /dev/sdb1 65536 64K ok
53 Run virt-alignment-scan without any -a or -d options to scan all
55 libvirt domains.
56 # virt-alignment-scan
58 F16x64:/dev/sda1 1048576 1024K ok
59 F16x64:/dev/sda2 2097152 2048K ok
60 F16x64:/dev/sda3 526385152 2048K ok
61 The output consists of 4 or more whitespace-separated columns. Only
63 the first 4 columns are significant if you want to parse this from a
64 program. The columns are:
65 col 1
67 The device and partition name (eg. "/dev/sda1" meaning the first
68 partition on the first block device).
69 When listing all libvirt domains (no -a or -d option given) this
71 column is prefixed by the libvirt name or UUID (if --uuid is
72 given). eg: "WinXP:/dev/sda1"
73 col 2
75 the start of the partition in bytes
76 col 3
78 the alignment in bytes or Kbytes (eg. 512 or "4K")
79 col 4
81 "ok" if the alignment is best for performance, or "bad" if the
82 alignment can cause performance problems
83 cols 5+
85 optional free-text explanation.
86 The exit code from the program changes depending on whether poorly
88 aligned partitions were found. See "EXIT STATUS" below.
89 If you just want the exit code with no output, use the -q option.
91
93 --help
94 Display brief help.
95 -a file
97 --add file
98 Add file which should be a disk image from a virtual machine.
99 The format of the disk image is auto-detected. To override this
101 and force a particular format use the --format=.. option.
102 -c URI
104 --connect URI
105 If using libvirt, connect to the given URI. If omitted, then we
106 connect to the default libvirt hypervisor.
107 If you specify guest block devices directly (-a), then libvirt is
109 not used at all.
110 -d guest
112 --domain guest
113 Add all the disks from the named libvirt guest. Domain UUIDs can
114 be used instead of names.
115 --format=raw|qcow2|..
117 --format
118 The default for the -a option is to auto-detect the format of the
119 disk image. Using this forces the disk format for -a options which
120 follow on the command line. Using --format with no argument
121 switches back to auto-detection for subsequent -a options.
122 For example:
124 virt-alignment-scan --format=raw -a disk.img
126 forces raw format (no auto-detection) for "disk.img".
128 virt-alignment-scan --format=raw -a disk.img --format -a another.img
130 forces raw format (no auto-detection) for "disk.img" and reverts to
132 auto-detection for "another.img".
133 If you have untrusted raw-format guest disk images, you should use
135 this option to specify the disk format. This avoids a possible
136 security problem with malicious guests (CVE-2010-3851).
137 -q
139 --quiet
140 Don't produce any output. Just set the exit code (see "EXIT
141 STATUS" below).
142 --uuid
144 Print UUIDs instead of names. This is useful for following a guest
145 even when the guest is migrated or renamed, or when two guests
146 happen to have the same name.
147 This option only applies when listing all libvirt domains (when no
149 -a or -d options are specified).
150 -v
152 --verbose
153 Enable verbose messages for debugging.
154 -V
156 --version
157 Display version number and exit.
158 -x Enable tracing of libguestfs API calls.
160
162 Operating systems older than Windows 2008 and Linux before ca.2010
163 place the first sector of the first partition at sector 63, with a 512
164 byte sector size. This happens because of a historical accident.
165 Drives have to report a cylinder / head / sector (CHS) geometry to the
166 BIOS. The geometry is completely meaningless on modern drives, but it
167 happens that the geometry reported always has 63 sectors per track.
168 The operating system therefore places the first partition at the start
169 of the second "track", at sector 63.
170 When the guest OS is virtualized, the host operating system and
172 hypervisor may prefer accesses aligned to one of:
173 · 512 bytes
175 if the host OS uses local storage directly on hard drive
177 partitions, and the hard drive has 512 byte physical sectors.
178 · 4 Kbytes
180 for local storage on new hard drives with 4Kbyte physical sectors;
182 for file-backed storage on filesystems with 4Kbyte block size; or
183 for some types of network-attached storage.
184 · 64 Kbytes
186 for high-end network-attached storage. This is the optimal block
188 size for some NetApp hardware.
189 · 1 Mbyte
191 see "1 MB PARTITION ALIGNMENT" below.
193 Partitions which are not aligned correctly to the underlying storage
195 cause extra I/O. For example:
196 sect#63
198 +--------------------------+------
199 | guest |
200 | filesystem block |
201 ---+------------------+------+-------------------+-----+---
202 | host block | host block |
203 | | |
204 ---+-------------------------+-------------------------+---
205 In this example, each time a 4K guest block is read, two blocks on the
207 host must be accessed (so twice as much I/O is done). When a 4K guest
208 block is written, two host blocks must first be read, the old and new
209 data combined, and the two blocks written back (4x I/O).
210 LINUX HOST BLOCK AND I/O SIZE
212 New versions of the Linux kernel expose the physical and logical block
213 size, and minimum and recommended I/O size.
214 For a typical consumer hard drive with 512 byte sectors:
216 $ cat /sys/block/sda/queue/hw_sector_size
218 512
219 $ cat /sys/block/sda/queue/physical_block_size
220 512
221 $ cat /sys/block/sda/queue/logical_block_size
222 512
223 $ cat /sys/block/sda/queue/minimum_io_size
224 512
225 $ cat /sys/block/sda/queue/optimal_io_size
226 0
227 For a new consumer hard drive with 4Kbyte sectors:
229 $ cat /sys/block/sda/queue/hw_sector_size
231 4096
232 $ cat /sys/block/sda/queue/physical_block_size
233 4096
234 $ cat /sys/block/sda/queue/logical_block_size
235 4096
236 $ cat /sys/block/sda/queue/minimum_io_size
237 4096
238 $ cat /sys/block/sda/queue/optimal_io_size
239 0
240 For a NetApp LUN:
242 $ cat /sys/block/sdc/queue/logical_block_size
244 512
245 $ cat /sys/block/sdc/queue/physical_block_size
246 512
247 $ cat /sys/block/sdc/queue/minimum_io_size
248 4096
249 $ cat /sys/block/sdc/queue/optimal_io_size
250 65536
251 The NetApp allows 512 byte accesses (but they will be very
253 inefficient), prefers a minimum 4K I/O size, but the optimal I/O size
254 is 64K.
255 For detailed information about what these numbers mean, see
257 http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Storage_Administration_Guide/newstorage-iolimits.html
258 [Thanks to Matt Booth for providing 4K drive data. Thanks to Mike
260 Snitzer for providing NetApp data and additional information.]
261 1 MB PARTITION ALIGNMENT
263 Microsoft picked 1 MB as the default alignment for all partitions
264 starting with Windows 2008 Server, and Linux has followed this.
265 Assuming 512 byte sectors in the guest, you will now see the first
267 partition starting at sector 2048, and subsequent partitions (if any)
268 will start at a multiple of 2048 sectors.
269 1 MB alignment is compatible with all current alignment requirements
271 (4K, 64K) and provides room for future growth in physical block sizes.
272 SETTING ALIGNMENT
274 virt-resize(1) can change the alignment of the partitions of some
275 guests. Currently it can fully align all the partitions of all Windows
276 guests, and it will fix the bootloader where necessary. For Linux
277 guests, it can align the second and subsequent partitions, so the
278 majority of OS accesses except at boot will be aligned.
279 Another way to correct partition alignment problems is to reinstall
281 your guest operating systems. If you install operating systems from
282 templates, ensure these have correct partition alignment too.
283 For older versions of Windows, the following NetApp document contains
285 useful information: http://media.netapp.com/documents/tr-3747.pdf
286 For Red Hat Enterprise Linux ≤ 5, use a Kickstart script that contains
288 an explicit %pre section that creates aligned partitions using
289 parted(8). Do not use the Kickstart "part" command. The NetApp
290 document above contains an example.
291
293 Libvirt guest names can contain arbitrary characters, some of which
294 have meaning to the shell such as "#" and space. You may need to quote
295 or escape these characters on the command line. See the shell manual
296 page sh(1) for details.
297
299 This program returns:
300 · 0
302 successful exit, all partitions are aligned ≥ 64K for best
304 performance
305 · 1
307 an error scanning the disk image or guest
309 · 2
311 successful exit, some partitions have alignment < 64K which can
313 result in poor performance on high end network storage
314 · 3
316 successful exit, some partitions have alignment < 4K which can
318 result in poor performance on most hypervisors
319
321 guestfs(3), guestfish(1), virt-filesystems(1), virt-rescue(1),
322 virt-resize(1), http://libguestfs.org/.
323
325 Richard W.M. Jones http://people.redhat.com/~rjones/
326
328 Copyright (C) 2011 Red Hat Inc.
329
331 This program is free software; you can redistribute it and/or modify it
332 under the terms of the GNU General Public License as published by the
333 Free Software Foundation; either version 2 of the License, or (at your
334 option) any later version.
335 This program is distributed in the hope that it will be useful, but
337 WITHOUT ANY WARRANTY; without even the implied warranty of
338 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
339 General Public License for more details.
340 You should have received a copy of the GNU General Public License along
342 with this program; if not, write to the Free Software Foundation, Inc.,
343 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
344
346 To get a list of bugs against libguestfs, use this link:
347 https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools
348 To report a new bug against libguestfs, use this link:
350 https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools
351 When reporting a bug, please supply:
353 · The version of libguestfs.
355 · Where you got libguestfs (eg. which Linux distro, compiled from
357 source, etc)
358 · Describe the bug accurately and give a way to reproduce it.
360 · Run libguestfs-test-tool(1) and paste the complete, unedited output
362 into the bug report.
363libguestfs-1.20.11 2013-08-27 virt-alignment-scan(1)