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Disk Mirroring Using Solaris Disk Manager
(RAID 1 volumes)

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RAID-1 volume in Solaris can be treated using two methods:

The syntax of metainit command is:

metainit mirror_volume -m submirror [read_options] [write_options] [pass_num]


As an example let's create a mirrored volume d10, and attach volume d11 as a one-way mirror using. After that let's attach volume d11 is a submirror of the mirrored volume d10.

# /usr/sbin/metainit d10 -m d11
d10: Mirror is setup

After creation of a mirror you need to update the /etc/vfstab file to change the mount point from a slice, such as /dev/dsk/c#t#d#s#, to a volume, such as /dev/md/dsk/d10.

metaroot device

where device specifies the mirrored volume used for the root file system.

The following example shows that the /etc/vfstab file has been updated by the metaroot command to point to the RAID-1 mirrored  metadevice.

# metaroot d10 && grep md /etc/vfstab

In addition to modifying the /etc/vfstab file to update the root (/) file system pointer, the metaroot command updates the /etc/system file to contain the forceload statement that loads the kernel modules that support the logical volumes. For example:

# tail /etc/system
forceload: misc/md_hotspares
forceload: misc/md_sp
forceload: misc/md_stripe
forceload: misc/md_mirror
forceload: drv/pcipsy
forceload: drv/simba
forceload: drv/glm
forceload: drv/sd

You must reboot the system before attaching the secondary submirror. Enter the init command to reboot the system using  reboot
After the reboot, attach the secondary submirror by using the metattach command:

# metattach d10 d12
d10: submirror d12 is attached

Note:  If the metattach command is not used, no resynchronization operations occur. As a result, data could become corrupted as the SVM assumes that both sides of the mirror are identical and can be used interchangeably.

If you mirror your root (/) file system, you also need to record the alternate boot path contained in the boot-device PROM variable. In the following example, you determine the path to the alternate boot device by using the ls -l command on the slice that is being attached as the secondary submirror to the root (/) mirror.

# ls -l /dev/dsk/c1t2d0s1
lrwxrwxrwx 1 root root 46 Feb 28 08:58 /dev/dsk/c1t2d0s1 -> ../../devices/pci@1f,0/pci@1/scsi@4,1/sd@2,0:b

Record the path that follows the /devices directory:


Note: When using old disk controllers in Ultra 5 and Ultra 10, the path to the device might differ between the entries in the /devices directory and the entries in the OpenBoot PROM. In these instances, follow the entries in the OpenBoot PROM. If you do not take into account existing difference and use entry from /devices  directory adapt to the you get an error:

can�t open boot device

To get the system to boot automatically from the alternate boot device in the event of a primary root submirror failure, complete the following steps:

  1. Use the OpenBoot nvalias command to define a backup_root device alias for the secondary root mirror. For example:

    ok nvalias backup_root /pci@1f,0/pci@1/scsi@4,1/disk@2,0:b

  2. Redefine the boot-device variable to reference both the primary and secondary submirrors, in the order in which you want to access them. For example:

    ok printenv boot-device
    boot-device= disk net
    ok setenv boot-device disk backup_root net
    boot-device= disk backup_root net


  3. To test the secondary submirror, boot the system manually, as follows:

    ok boot backup_root

If system can book OK then in the event of primary root disk failure, the system will automatically try to boot from the secondary submirror.

Unmirroring the root (/) File System

Run the metastat command on the mirror to verify that submirror 0 is in the Okay state, for example:

# metastat d10

2. Run the metadetach command on the mirror to make a one-way mirror.

# metadetach d10 d12
d10: submirror d12 is detached

3. Because this is a root (/) file system mirror, run the metaroot command again in order to update the /etc/vfstab and /etc/system files.

# metaroot /dev/dsk/c0t0d0s0 && grep c0t0d0s0 /etc/vfstab

4. Reboot the system using reboot

5. Run the metaclear command to clear the mirror and submirrors. The -r option recursively deletes specified metadevices and hot spare pools, associated with the targeted metadevices specified in the metaclear command. For example

# metaclear -r d10
d10: Mirror is cleared
d11: Concat/Stripe is cleared
# metaclear d12
d12: Concat/Stripe is cleared

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Since RAID0 improves performance, and RAID1 provides redundancy, someone came up with the idea to combine them. Fast and reliable. Two great tastes that taste great together!

When combining these two types of 'logical' devices there's a choice to be made -- do you mirror two stripes, or do you stripe across multiple mirrors? There are pros and cons to each approach:

SVM specifics

So, does SVM do RAID 0+1 or RAID 1+0? The answer is, "Yes." So it gives you a choice between the two? The answer is "No."

Obviously further explanation is necessary...

In SVM, mirror devices cannot be created from "bare" disks. You are required to create the mirror on top of another type of SVM metadevice, known as a concat/stripe*. SVM combines concatenations and stripes into a single metadevice type, in which one or more stripes are concatenated together. When used to build a mirror these concat/stripe logical devices are known as submirrors. If you want to expand the size of a mirror device you can do so by concatenating additional stripe(s) onto the concat/stripe devices that are serving as submirrors.

So, in SVM, you are always required to set up a stripe (concat/stripe) in order to create a mirror. On the surface this makes it appear that SVM does RAID 0+1. However, once you understand a bit about the SVM mirror code, you'll find RAID 1+0 lurking under the covers.

SVM mirrors are logically divided up into regions. The state of each mirror region is recorded in state database replicas* stored on disk. By individually recording the state of each region in the mirror, SVM can be smart about how it performs a resync. Following a disk failure or an unusual event (e.g. a power failure occurs after the first side of a mirror has been written to but before the matching write to the second side can be accomplished), SVM can determine which regions are out-of-sync and only synchronize them, not the entire mirror. This is known as an optimized resync.

The optimized resync mechanisms allow SVM to gain the redundancy benefits of RAID 1+0 while keeping the administrative benefits of RAID 0+1. If one of the drives in a concat/stripe device fails, only those mirror regions that correspond to data stored on the failed drive will lose redundancy. The SVM mirror code understands the layout of the concat/stripe submirrors and can therefore determine which resync regions reside on which underlying devices. For all regions of the mirror not affected by the failure, SVM will continue to provide redundancy, so a second disk failure won't necessarily prove fatal.

So, in a nutshell, SVM provides a RAID 0+1 style administrative interface but effectively implements RAID 1+0 functionality. Administrators get the best of each type, the relatively simple administration of RAID 0+1 plus the greater resilience of RAID 1+0 in the case of multiple device failures.

* concat/stripe logical devices (metadevices)

The following example shows a concat/stripe metadevice that's serving as a submirror to a mirror metadevice. Note that the metadevice is a concatenation of three separate stripes:

** State database replicas

SVM stores configuration and state information in a 'state database' in memory. Copies of this state database are stored on disk, where they are referred to as state database replicas. The primary purpose of the state database replicas is to provide non-volatile copies of the state database so that the SVM configuration is persistant across reboots. A secondary purpose of the replicas is to provide a 'scratch pad' to keep track of mirror region states.

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Compelling new features such as soft partitioning and automatic device relocation make the Solaris Volume Manager software a viable candidate for storage management needs. Solaris Volume Manager software features enhance storage management capabilities beyond what is handled by intelligent storage arrays with hardware RAID. Now Solaris Volume Manager software is integrated with the Solaris Operating Environment (Solaris OE) and does not require additional license fees. This article provides specific Solaris Volume Manager tips for system, storage, and database administrators who want get the most of Solaris Volume Manager software in their data centers. This article targets an intermediate audience.
Forcedirectio optimization (please read Solaris Volume Manager Performance Best Practices before using it)
forcedirectio | noforcedirectio
If forcedirectio is specified and supported by the file system, then for the duration of the mount, forced direct I/O will be used. If the filesystem is mounted using forcedirectio, data is transferred directly between user address space and the disk. If the filesystem is mounted using noforcedirectio, data is buffered in kernel address space when data is transferred between user address space and the disk. forcedirectio is a performance option that is of benefit only in large sequential data transfers. The default behavior is noforcedirectio.

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