1virt-alignment-scan(1) Virtualization Support virt-alignment-scan(1)
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6 virt-alignment-scan - Check alignment of virtual machine partitions
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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 -a URI
104 --add URI
105 Add a remote disk. See "ADDING REMOTE STORAGE" in guestfish(1).
106 --blocksize=512
108 --blocksize=4096
109 --blocksize
110 This parameter sets the sector size of the disk image. It affects
111 all explicitly added subsequent disks after this parameter. Using
112 --blocksize with no argument switches the disk sector size to the
113 default value which is usually 512 bytes. See also
114 "guestfs_add_drive_opts" in guestfs(3).
115 -c URI
117 --connect URI
118 If using libvirt, connect to the given URI. If omitted, then we
119 connect to the default libvirt hypervisor.
120 If you specify guest block devices directly (-a), then libvirt is
122 not used at all.
123 -d guest
125 --domain guest
126 Add all the disks from the named libvirt guest. Domain UUIDs can
127 be used instead of names.
128 --format=raw|qcow2|..
130 --format
131 The default for the -a option is to auto-detect the format of the
132 disk image. Using this forces the disk format for -a options which
133 follow on the command line. Using --format with no argument
134 switches back to auto-detection for subsequent -a options.
135 For example:
137 virt-alignment-scan --format=raw -a disk.img
139 forces raw format (no auto-detection) for disk.img.
141 virt-alignment-scan --format=raw -a disk.img --format -a another.img
143 forces raw format (no auto-detection) for disk.img and reverts to
145 auto-detection for another.img.
146 If you have untrusted raw-format guest disk images, you should use
148 this option to specify the disk format. This avoids a possible
149 security problem with malicious guests (CVE-2010-3851).
150 -P nr_threads
152 Since libguestfs 1.22, virt-alignment-scan is multithreaded and
153 examines guests in parallel. By default the number of threads to
154 use is chosen based on the amount of free memory available at the
155 time that virt-alignment-scan is started. You can force virt-
156 alignment-scan to use at most "nr_threads" by using the -P option.
157 Note that -P 0 means to autodetect, and -P 1 means to use a single
159 thread.
160 -q
162 --quiet
163 Don’t produce any output. Just set the exit code (see "EXIT
164 STATUS" below).
165 --uuid
167 Print UUIDs instead of names. This is useful for following a guest
168 even when the guest is migrated or renamed, or when two guests
169 happen to have the same name.
170 This option only applies when listing all libvirt domains (when no
172 -a or -d options are specified).
173 -v
175 --verbose
176 Enable verbose messages for debugging.
177 -V
179 --version
180 Display version number and exit.
181 -x Enable tracing of libguestfs API calls.
183
185 Operating systems older than Windows 2008 and Linux before ca.2010
186 place the first sector of the first partition at sector 63, with a 512
187 byte sector size. This happens because of a historical accident.
188 Drives have to report a cylinder / head / sector (CHS) geometry to the
189 BIOS. The geometry is completely meaningless on modern drives, but it
190 happens that the geometry reported always has 63 sectors per track.
191 The operating system therefore places the first partition at the start
192 of the second "track", at sector 63.
193 When the guest OS is virtualized, the host operating system and
195 hypervisor may prefer accesses aligned to one of:
196 • 512 bytes
198 if the host OS uses local storage directly on hard drive
200 partitions, and the hard drive has 512 byte physical sectors.
201 • 4 Kbytes
203 for local storage on new hard drives with 4Kbyte physical sectors;
205 for file-backed storage on filesystems with 4Kbyte block size; or
206 for some types of network-attached storage.
207 • 64 Kbytes
209 for high-end network-attached storage. This is the optimal block
211 size for some NetApp hardware.
212 • 1 Mbyte
214 see "1 MB PARTITION ALIGNMENT" below.
216 Partitions which are not aligned correctly to the underlying storage
218 cause extra I/O. For example:
219 sect#63
221 ┌──────────────────────────┬ ─ ─ ─ ─
222 │ guest │
223 │ filesystem block │
224 ─ ┬──────────────────┴──────┬───────────────────┴─────┬ ─ ─
225 │ host block │ host block │
226 │ │ │
227 ─ ┴─────────────────────────┴─────────────────────────┴ ─ ─
228 In this example, each time a 4K guest block is read, two blocks on the
230 host must be accessed (so twice as much I/O is done). When a 4K guest
231 block is written, two host blocks must first be read, the old and new
232 data combined, and the two blocks written back (4x I/O).
233 LINUX HOST BLOCK AND I/O SIZE
235 New versions of the Linux kernel expose the physical and logical block
236 size, and minimum and recommended I/O size.
237 For a typical consumer hard drive with 512 byte sectors:
239 $ cat /sys/block/sda/queue/hw_sector_size
241 512
242 $ cat /sys/block/sda/queue/physical_block_size
243 512
244 $ cat /sys/block/sda/queue/logical_block_size
245 512
246 $ cat /sys/block/sda/queue/minimum_io_size
247 512
248 $ cat /sys/block/sda/queue/optimal_io_size
249 0
250 For a new consumer hard drive with 4Kbyte sectors:
252 $ cat /sys/block/sda/queue/hw_sector_size
254 4096
255 $ cat /sys/block/sda/queue/physical_block_size
256 4096
257 $ cat /sys/block/sda/queue/logical_block_size
258 4096
259 $ cat /sys/block/sda/queue/minimum_io_size
260 4096
261 $ cat /sys/block/sda/queue/optimal_io_size
262 0
263 For a NetApp LUN:
265 $ cat /sys/block/sdc/queue/logical_block_size
267 512
268 $ cat /sys/block/sdc/queue/physical_block_size
269 512
270 $ cat /sys/block/sdc/queue/minimum_io_size
271 4096
272 $ cat /sys/block/sdc/queue/optimal_io_size
273 65536
274 The NetApp allows 512 byte accesses (but they will be very
276 inefficient), prefers a minimum 4K I/O size, but the optimal I/O size
277 is 64K.
278 For detailed information about what these numbers mean, see
280 http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Storage_Administration_Guide/newstorage-iolimits.html
281 [Thanks to Matt Booth for providing 4K drive data. Thanks to Mike
283 Snitzer for providing NetApp data and additional information.]
284 1 MB PARTITION ALIGNMENT
286 Microsoft picked 1 MB as the default alignment for all partitions
287 starting with Windows 2008 Server, and Linux has followed this.
288 Assuming 512 byte sectors in the guest, you will now see the first
290 partition starting at sector 2048, and subsequent partitions (if any)
291 will start at a multiple of 2048 sectors.
292 1 MB alignment is compatible with all current alignment requirements
294 (4K, 64K) and provides room for future growth in physical block sizes.
295 SETTING ALIGNMENT
297 virt-resize(1) can change the alignment of the partitions of some
298 guests. Currently it can fully align all the partitions of all Windows
299 guests, and it will fix the bootloader where necessary. For Linux
300 guests, it can align the second and subsequent partitions, so the
301 majority of OS accesses except at boot will be aligned.
302 Another way to correct partition alignment problems is to reinstall
304 your guest operating systems. If you install operating systems from
305 templates, ensure these have correct partition alignment too.
306 For older versions of Windows, the following NetApp document contains
308 useful information: http://media.netapp.com/documents/tr-3747.pdf
309 For Red Hat Enterprise Linux ≤ 5, use a Kickstart script that contains
311 an explicit %pre section that creates aligned partitions using
312 parted(8). Do not use the Kickstart "part" command. The NetApp
313 document above contains an example.
314
316 This program returns:
317 • 0
319 successful exit, all partitions are aligned ≥ 64K for best
321 performance
322 • 1
324 an error scanning the disk image or guest
326 • 2
328 successful exit, some partitions have alignment < 64K which can
330 result in poor performance on high end network storage
331 • 3
333 successful exit, some partitions have alignment < 4K which can
335 result in poor performance on most hypervisors
336
338 guestfs(3), guestfish(1), virt-filesystems(1), virt-rescue(1),
339 virt-resize(1), http://libguestfs.org/.
340
342 Richard W.M. Jones http://people.redhat.com/~rjones/
343
345 Copyright (C) 2011 Red Hat Inc.
346
348 This program is free software; you can redistribute it and/or modify it
349 under the terms of the GNU General Public License as published by the
350 Free Software Foundation; either version 2 of the License, or (at your
351 option) any later version.
352 This program is distributed in the hope that it will be useful, but
354 WITHOUT ANY WARRANTY; without even the implied warranty of
355 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
356 General Public License for more details.
357 You should have received a copy of the GNU General Public License along
359 with this program; if not, write to the Free Software Foundation, Inc.,
360 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
361
363 To get a list of bugs against libguestfs, use this link:
364 https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools
365 To report a new bug against libguestfs, use this link:
367 https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools
368 When reporting a bug, please supply:
370 • The version of libguestfs.
372 • Where you got libguestfs (eg. which Linux distro, compiled from
374 source, etc)
375 • Describe the bug accurately and give a way to reproduce it.
377 • Run libguestfs-test-tool(1) and paste the complete, unedited output
379 into the bug report.
380guestfs-tools-1.51.6 2023-12-09 virt-alignment-scan(1)