virt-alignment-scan - Check alignment of virtual machine partitions
virt-alignment-scan [--options] -d domname
virt-alignment-scan [--options] -a disk.img [-a disk.img ...]
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 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: http://media.netapp.com/documents/tr-3747.pdf
To run this tool on a disk image directly, use the -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 -d option and possibly the -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 significant if you want to parse this from a program. The columns are:
the device and partition name (eg. /dev/sda1 meaning the
first partition on the first block device)
the start of the partition in bytes
the alignment in bytes or Kbytes (eg. 512 or 4K)
ok if the alignment is best for performance, or bad if the
alignment can cause performance problems
optional free-text explanation.
The exit code from the program changes depending on whether poorly aligned partitions were found. See EXIT STATUS below.
If you just want the exit code with no output, use the -q option.
Display brief help.
Add 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 --format=.. option.
If using libvirt, connect to the given URI. If omitted, then we connect to the default libvirt hypervisor.
If you specify guest block devices directly (-a), then libvirt is not used at all.
Add all the disks from the named libvirt guest. Domain UUIDs can be used instead of names.
The default for the -a option is to auto-detect the format of the disk image. Using this forces the disk format for -a options which follow on the command line. Using --format with no argument switches back to auto-detection for subsequent -a options.
For example:
virt-alignment-scan --format=raw -a disk.img
forces raw format (no auto-detection) for disk.img.
virt-alignment-scan --format=raw -a disk.img --format -a another.img
forces raw format (no auto-detection) for disk.img and reverts to
auto-detection for 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).
Don't produce any output. Just set the exit code (see EXIT STATUS below).
Enable verbose messages for debugging.
Display version number and exit.
Enable tracing of libguestfs API calls.
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:
if the host OS uses local storage directly on hard drive partitions, and the hard drive has 512 byte physical sectors.
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.
for high-end network-attached storage. This is the optimal block size for some NetApp hardware.
see 1 MB PARTITION ALIGNMENT below.
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).
New versions of the Linux kernel expose the physical and logical block size, and minimum and recommended I/O size.
For a typical consumer hard drive with 512 byte sectors:
$ cat /sys/block/sda/queue/hw_sector_size 512 $ 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 new consumer hard drive with 4Kbyte sectors:
$ cat /sys/block/sda/queue/hw_sector_size 4096 $ cat /sys/block/sda/queue/physical_block_size 4096 $ cat /sys/block/sda/queue/logical_block_size 4096 $ cat /sys/block/sda/queue/minimum_io_size 4096 $ 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 http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Storage_Administration_Guide/newstorage-iolimits.html
[Thanks to Matt Booth for providing 4K drive data. Thanks to Mike Snitzer for providing NetApp data and additional information.]
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.
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: http://media.netapp.com/documents/tr-3747.pdf
For Red Hat Enterprise Linux ≤ 5, use a Kickstart script that
contains an explicit %pre section that creates aligned partitions
using parted(8). Do not use the Kickstart part command. The
NetApp document above contains an example.
Libvirt guest names can contain arbitrary characters, some of which
have meaning to the shell such as # and space. You may need to
quote or escape these characters on the command line. See the shell
manual page sh(1) for details.
This program returns:
0
successful exit, all partitions are aligned ≥ 64K for best performance
1
an error scanning the disk image or guest
2
successful exit, some partitions have alignment < 64K which can result in poor performance on high end network storage
3
successful exit, some partitions have alignment < 4K which can result in poor performance on most hypervisors
guestfs(3), guestfish(1), virt-filesystems(1), virt-rescue(1), virt-resize(1), http://libguestfs.org/.
Richard W.M. Jones http://people.redhat.com/~rjones/
Copyright (C) 2011 Red Hat Inc.
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