xlv(7M)xlv(7M)NAMExlv - logical volume disk driver
SYNOPSIS
/dev/xlv/*
/dev/rxlv/*
DESCRIPTION
XLV devices provide access to disk storage as logical volumes. A logical
volume is an object that behaves like a disk partition, but its storage
can span several physical disk devices.
Using XLV, you can concatenate disks together to create larger logical
volumes, stripe data across disks to create logical volumes with greater
throughput, and plex (or mirror) disks for reliability. In addition, XLV
enables you to change the configuration of volumes while the volume is
actively being used as a filesystem.
The geometry of logical volumes (e.g., the disks that belong to it, how
they are put together, etc.) are stored in the disk labels of the disks
that belong to the logical volumes. When the system starts up, the
utility xlv_assemble(1M) scans all the disks on the system and
automatically assembles them into logical volumes. xlv_assemble(1M) also
creates any necessary device nodes.
XLV device names always begin with /dev/{r}xlv/device_name where the
device_name is assigned by the creator of the volume. See xlv_make(1M)
for how volumes are created.
Device numbers range from 0 to one less than the maximum number of
logical volume devices configured in the system. This is 10 by default;
this number can be changed by rebuilding a kernel with lboot(1M).
There is a kernel driver, referred to as xlv, and some daemons for the
logical volume devices. The driver is a 'pseudo device' not directly
associated with any physical hardware; its function is to map requests on
logical volume devices into requests on the underlying disk devices. The
daemons take care of error recovery and dynamic reconfiguration of
volumes.
Volume Objects
XLV allows you to work with whole volumes and pieces of volumes. Pieces
of volumes are useful for creating and reconfiguring volumes in units
that are larger than individual disk partitions.
Each volume consists of up to three subvolumes. An xfs(4) filesystem
usually has a large data subvolume in which all the user files and
metadata such as inodes are stored and a small log subvolume in which the
filesystem log is stored. For high-performance and real-time
applications, a volume can also have a real-time subvolume that contains
only user files aligned at configurable block boundaries. Guaranteed
rate I/O can be done to real-time subvolumes. See grio(5).
Page 1
xlv(7M)xlv(7M)
Each subvolume can be independently organized as 1 to 4 plexes. Plexes
are sometimes known as mirrors. XLV makes sure that the data in all the
plexes of a subvolume are the same. Plexes are useful for reliability
since a subvolume remains available if any of its plexes are available.
Since each subvolume is independently organized, you can choose to plex
any, all, or none of the subvolumes within a volume.
Each plex consists of up to 128 volume elements. Each volume element is
a collection of disk partitions that may be either striped or
concatenated. By adding volume elements, you can extend the size of a
subvolume -- even one that is striped. Volume elements within a plex do
not need to be of the same size. However, all the volume elements at the
same offset in all the plexes of the subvolume must be the same size.
For example, the first and second volume elements in a plex can have
different sizes. But the first volume element in all the plexes of the
subvolume must be the same size. This restriction is necessary because
the volume element is the unit of recovery. Note that if XLV gets an
unrecoverable disk error on one disk partition in a volume element, the
entire volume element is taken offline.
Each volume element can consist of from 1 to 100 disk partitions. The
disks can be treated as either a concatenated set (in which case XLV
writes to the partitions sequentially) or as a striped set (in which case
XLV writes a stripe unit's worth of data to one disk and then rotates to
the next disk in the stripe set.) In general, it is better to use volume
elements that contain single disks when you want to concatenate disks
together and only use volume elements with multiple disks when you want
to use disk-striping. This is because the volume element is the unit of
recovery.
XLV allows you to create and work with volumes, subvolumes, plexes, and
volume elements. The interesting operations associated with volumes are:
creating them, assembling disk partitions into volumes, mounting them,
changing volume configurations, shutting them down, and destroying them.
Naming Volume Objects
Each XLV object is composed of a hierarchy of lower level objects. For
example, a volume is composed of subvolumes that are in turn composed of
plexes, etc. To let you refer to a component of an XLV object, XLV has
adopted a hierarchical naming convention. For example:
movies.data.0.5.50 Refers to the volume named movie, the data subvolume,
plex 0 of that subvolume, volume element 5 within
that plex, and disk partition 50 within that volume
element. Note that the numbers are zero-based.
movies.log.2 Refers to plex number 2 in the log subvolume of the
volume named movies.
movies.rt.1.5 Refers to volume element 5 within plex number 1 of
the real-time subvolume of the volume named movies.
Page 2
xlv(7M)xlv(7M)
If you create an object outside of a volume, then that object has a
user-assigned name. For example, spare_plex.2.1 refers to disk partition
number 1 of volume element number 2 of a standalone plex named
spare_plex. spare_plex does not currently belong to any subvolumes.
These names are echoed by xlv_make(1M) as objects are created. They are
also useful in specifying the objects to change via xlv_mgr(1M).
Creating Volumes
Volumes are created via xlv_make(1M). This utility writes the volume
geometry to all the disks that belong to the volume object. The geometry
is written to the volume headers. See vh(7M).
Assembling Volumes
After a volume has been created, it must be made known to the kernel
driver before I/O can be initiated to the volume. The command
xlv_assemble(1M) scans all the disks attached to the system and assembles
all the logical volumes that it finds. It then passes the configuration
to the kernel. This is usually done during system startup. Once a
volume has been assembled, I/O can be performed.
Working with Filesystems
The normal filesystem utilities such as mkfs(1M) and mount(1M) work with
logical volumes.
A logical volume consisting of a single disk partition (that may be
plexed) can be used as root(7M). You cannot boot directly off a logical
volume; you must specify the underlying disk partition.
Modifying Volumes
The geometry of a volume object can be modified either offline or online.
To modify a volume object offline, first unmount the filesystem, then
destroy the volume object by using xlv_mgr(1M). Then, you can run
xlv_make(1M) to create new XLV objects. Note that xlv_make only allows
you to use disk partitions that are not currently part of volume objects.
You can also modify volume objects while they are online by using
xlv_mgr(1M). You can grow a volume, add a plex, and remove a plex while
the volume is actively being used. Note that I/O is blocked while the
configuration is being changed. The blocked I/O is completed after the
configuration has been written out to the disk labels.
You can also use xlv_mgr to remove a volume element from a plex while the
volume is online if there is at least one other plex that covers the
range of disk blocks affected. Note that you can choose to plex only a
portion of the address space of a subvolume.
Working with Plexes
When there are multiple plexes, XLV recovers from read errors. In
addition, XLV attempts to rewrite the data back to the failed plex. XLV
masks write errors if it can write to at least one of the plexes.
Page 3
xlv(7M)xlv(7M)
When a plexed volume starts up, XLV automatically makes sure that all the
data among the plexes within each subvolume is consistent. This may
involve copying the data from one plex to the others. While this is
going on, the volume is available at a degraded performance. You can
eliminate the need for plex recovery by shutting down the plex with
xlv_shutdown(1M). xlv_shutdown synchronizes the plexes and marks them as
been the same so that when they restart, XLV knows that the plexes are
consistent and can therefore avoid the plex copies.
FILES
/dev/xlv/*
/dev/rxlv/*
/var/sysgen/master.d/xlv
SEE ALSOcfg(1M), lv_to_xlv(1M), xlv_assemble(1M), xlv_labd(1M), xlv_make(1M),
xlv_mgr(1M), xlv_plexd(1M), xlv_shutdown(1M), xlvd(1M), grio(5).
NOTES
XLV runs on both XFS and EFS filesystems. In addition, you can read and
write to XLV devices using the raw device interfaces.
XLV disk labels are stored on the disks themselves. Therefore, you can
physically reposition the disk drives and XLV still assembles them
correctly.
You can upgrade from an existing lv(7M) volume to an XLV volume by using
lv_to_xlv(1M).
When you are running in the miniroot, the XLV device nodes are created in
/root/dev/xlv and /root/dev/rxlv.
Page 4
