Solaris Volume Manager (SVM)

The Solaris volume manager (SVM) is a free component of  Solaris 9 and Solaris 10. It was previously known as Solstice DiskSuite. SVM software provides mechanisms to configure physical slices of harddrive into logical volumes. As such it introduces additional level of complexity and should not be used unless absolutely necessary. On Solaris 10 using ZFS is a better alternative. Logical volumes can be configured to provide mirroring and RAID5.  In its simplest form SVM uses traditional Solaris disk partitioning (up to eight partitions or slices in Solaris terminology) to build virtual disks called volumes.

Any partition can be used to create volumes, but it is common practice to reserve slice s7 for the state database replicas. Database replicas are created on selected disks and hold the SVM configuration data. It is the administrator’s responsibility to create these state databases (using the metadb command) and distribute them across disks and controllers to avoid any single points of failure.

The version of SVM shipped with Solaris 9 has several new features: soft partitions, monitoring active disks, access via SMC.

The soft partitioning feature allows to subdivide a disk into many small slices that are controlled and maintained by software (hence the term soft partitioning). Soft partitioning allows up to 8192 partitions on a single drive or volume, providing greater flexibility as with today’s 500G disks, customers often need to subdivide a disk into more then 8 partitions. Solaris Volume Manager enables an administrator to create soft partitions either on top of individual physical disks, or on existing hardware RAID 1, RAID 5, or RAID 0 volumes.

All mirrored volumes in SVM automatically benefit from volume logging. Volume logging limits the amount of block copy activity necessary to keep the mirrored volumes in sync. Volume logging uses bitmaps held in the state databases to track changes to submirrors. Consequently, all mirrored volumes are protected against the need to perform a full-mirror resynchronization in the event of a system failure.

Tips:

• A state database replica must be created on the disk partition before a UFS file system is created.
• Default side of state database replica is 4M.
• The Solaris Volume Manager software provides a GUI called the Enhanced Storage Tool to perform volume management tasks
   
• RAID 0 striping is usually not that efficient with modern disks with their complex hardware and controller caches.  The return on the investment might be questionable. Usage of higher speed disk (15K RPM instead of  10K RPM ) might be a better way to increase read speed. Also consider NAS or "poor man NAS": concatenation of slices positioned on multiple smaller disks preferably connected to different controllers (for example 4 36G connected to 2 two-channel controllers or 8 18G drives connected to 4 two channel controllers instead of one 128G disk). Compatible LSI controllers can be bought pretty cheap on eBay.
   
• RAID 1 volumes (mirroring) can benefit from increased sync buffer size to 1M (2048 512 blocks). To experiment use metasync -r 2048 command. For permanent changes:
  • On the Solaris 10 systems added the following line to /etc/system.

    set md_mirror:md_resync_bufsz = 2048
     

  • For Solaris 9 edit the S95svm.sync file to include the "2048" buffer size.
     
• RAID 5 volumes:
  • The system must contain at least three state database replicas before you can create RAID5 volumes.
  • A RAID5 volume can only handle a single slice failure.
  • Follow the 20-percent rule when creating a RAID5 volume: because of the complexity of parity calculations, volumes with greater than about 20 percent writes should probably not be RAID5 volumes. If data redundancy is needed, consider mirroring.
     

    Volumes with greater than about 20 percent of write operations should probably not be RAID5 volumes

Summary:

 metadb -a -- creates state database replicas  (you can also create then using The Solaris Volume Manager software GUI) The syntax of the command is:

metadb -a [-f] [-c n] [-l nnnn] disk_slice

metainit -- create RAID volumes The force (-f) option must be used because this is the root (/) file system, which cannot be unmounted. The syntax of the metainit command is:

metainit -f [concat_vol | stripe_vol] numstripes width component...

metastat -- Verify the status of the root (/) submirrors.

# /usr/sbin/metastat d10

metaroot allow mirroring the root (/) file system, modifes the /etc/vfstab and /etc/system files, as follows:

metaroot device

metadetach -- Detach one submirror to make the root (/) mirror a one-way mirror.

# /usr/sbin/metadetach d10 d12

metaclear 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.

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

NEWS CONTENTS

• 20060114 : Using Solaris Volume Manager ( Using Solaris Volume Manager, Jan 14, 2006 )
• 20060114 : Andre Molyneuxs Weblog ( Andre Molyneux's Weblog , )

Old News ;-)

[Jan 14, 2006] Using Solaris Volume Manager -- a good overview

The Solaris Volume Manager (SVM) is packaged with Solaris 9 and provides advanced hard disk management capabilities, including creating RAID volumes, soft partitions, hot spare pools, and transactional volumes. Using SVM can help increase your storage capacity, ensure data availability, improve disk I/O, and lower administrative overhead. SVM was formerly known as the Solstice DiskSuite.

Andre Molyneux's Weblog

• RAID0 is striping, in which data is spread across multiple spindles (disks). Data written to the RAID0 is broken up into chunks of a specified size (interlace value) and spread across the disks:

  +--------+     +--------+
  | Chunk 1|     |Chunk 2 |
  +--------+     +--------+
  | Chunk 3|     |Chunk 4 |
  +--------+     +--------+
  | Chunk 5|     |Chunk 6 |
  +--------+     +--------+
    Disk A         Disk B
  |                       |
  +--------Stripe---------+
  

  In this case the term RAID is a misnomer as no redundancy is gained. The address space is 'interlaced' across the disks, improving performance for I/Os bigger than the interlace/chunk size as a single I/O will spread across multiple disks. In the example above, for an interlace size of 16k, the first 16k of data would reside on Disk A, the second 16k on Disk B, the third 16k on Drive A, and so on. The interlace size needs to be chosen when the logical device is created and can't be changed later without recreating the logical device from scratch.
  

• RAID1: RAID1 is mirroring. Every byte of data written to the mirror is duplicated on both disks:

  +--------+     +--------+
  | Copy A | <-> | Copy B |
  +--------+     +--------+
    Disk A         Disk B
  |                       |
  +--------Mirror---------+
  

  The advantages are that you can lose either disk and the data will still be accessible, and reads can be alternated between the two disks to improve performance. The drawbacks are that you've doubled your storage costs and incurred additional overhead by having to generate two writes to physical devices for every one that's done to the logical mirror device.

• Concatenation: One additional type of logical device that's involved when combining stripes and mirrors in SVM is a concatenation. There is no RAID nomencalture associated with a 'concat':

  +--------+
  | Disk A |
  +--------+
  | Disk B |
  +--------+
  | Disk C |
  +--------+
    Concat
  

  A concatenation aggregates several smaller physical devices into one large logical device. Unlike a stripe, the address space isn't interlaced across the underlying devices. This means there's no performance gain from using a concatenated device.

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:

• RAID 0+1: In RAID 0+1, the stripes are created first, then are mirrored together. Logically, the resulting device looks like:

  +------------------------------------+     +------------------------------------+
  | +--------+  +--------+  +--------+ |     | +--------+  +--------+  +--------+ |
  | | Chunk 1|  |Chunk 2 |  |Chunk 3 | |     | | Chunk 1|  |Chunk 2 |  |Chunk 3 | |
  | +--------+  +--------+  +--------+ |     | +--------+  +--------+  +--------+ |
  | | Chunk 4|  |Chunk 5 |  |Chunk 6 | |     | | Chunk 4|  |Chunk 5 |  |Chunk 6 | |
  | +--------+  +--------+  +--------+ |<--->| +--------+  +--------+  +--------+ |
  | | Chunk 7|  |Chunk 8 |  |Chunk 9 | |     | | Chunk 7|  |Chunk 8 |  |Chunk 9 | |
  | +--------+  +--------+  +--------+ |     | +--------+  +--------+  +--------+ |
  |   Disk A      Disk B      Disk C   |     |    Disk D     Disk E      Disk F   |
  +------------------------------------+     +------------------------------------+
           Stripe 1                                        Stripe 2
  |                                                                               |
  +-----------------------------------Mirror--------------------------------------+
  

  Advantage: Simple administrative model. Issue one command to create the first stripe, a second command to create the second stripe, and a third command to mirror them. Three commands and you're done, regardless of the number of disks in the configuration.
  Disadvantage: An error on any one of the disks kills redundancy for all disks. For instance, a failure on Disk B above 'breaks' the Stripe 1 side of the mirror. As a result, should disk D, E, or F fail as well, the entire mirror becomes unusable.

• RAID 1+0: In RAID 1+0 the opposite approach is taken. The disks are mirrored first, then the mirrors are combined together into a stripe:

  +-----------------+  +-----------------+  +-----------------+
  |    Chunk 1      |  |     Chunk 2     |  |     Chunk 3     |
  +-----------------+  +-----------------+  +-----------------+
  |    Chunk 4      |  |     Chunk 5     |  |     Chunk 6     |
  +-----------------+  +-----------------+  +-----------------+
  |    Chunk 7      |  |     Chunk 8     |  |     Chunk 9     |
  +-----------------+  +-----------------+  +-----------------+
  Disk A <---> Disk B  Disk C <---> Disk D  Disk E <---> Disk F
        Mirror 1             Mirror 2             Mirror 3
  |                                                           |
  +-----------------------------------------------------------+
                            Stripe
  

  Advantage: A failure on one disk only impacts redundancy for the chunks of the stripe that are located on that disk. For instance, a failure on Disk B above only loses redundancy for every third chunk (1, 4, 7, etc.) Redundancy for the other stripe chunks is unaffected, so a second disk failure could be tolerated as long as the second failure wasn't on Disk A.
  Disadvantage: More complicated from an administrative standpoint. The administrator needs to issue one creation command per mirror, then a command to stripe across the mirrors. The six-disk example above would require four commands to create, while a twelve disk configuration would require seven commands.

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:

• Stripe 0 is a 1-way stripe (so not really striped at all) on disk slice c1t11d0s0.
• Stripe 1 is a 1-way stripe on disk slice c1t12d0s0.
• Stripe 2 is a 2-way stripe with an interlace size of 32 blocks on disk slices c1t13d0s1 and c1t14d0s2.

d1: Submirror of d0
    State: Okay
    Size: 78003 blocks (38 MB)
    Stripe 0:
        Device      Start Block  Dbase        State Reloc Hot Spare
        c1t11d0s0          0     No            Okay   Yes
    Stripe 1:
        Device      Start Block  Dbase        State Reloc Hot Spare
        c1t12d0s0          0     No            Okay   Yes
    Stripe 2: (interlace: 32 blocks)
        Device      Start Block  Dbase        State Reloc Hot Spare
        c1t13d0s1          0     No            Okay   Yes
        c1t14d0s2          0     No            Okay   Yes

** 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.

Recommended Links

Softpanorama Recommended

Top articles

Sites

• [PDF] Transitioning to Solaris™ Volume Manager - Sun Microsystems
• [PDF] Solaris Volume Manager Performance Best Practices
• docs.sun.com Solaris 9 Installation Guide
• docs.sun.com: Solaris Volume Manager Administration Guide
• Solaris Tips and Tricks
• Solaris Volume Manager Performance Best Practices
• Sun Solaris Solaris Volume Manager - (Solaris 9)

Recommended Papers

Sun Solaris Solaris Volume Manager - (Solaris 9)

Solaris Volume Manager Data Sheet - (Solaris 9)
Introduction
Installing
Command Line Tools
Starting the Volume Manager Console GUI - (Enhanced Storage)
Volume Manager Components
Creating Volumes - (Using Solaris 9 Volume Manager Commands)
State Database - (State Database Replicas)
Creating a Stripe - (RAID 0)
Creating a Concatenation - (RAID 0)
Creating Mirrors - (RAID 1)
Creating a RAID5 Volume - (RAID 5)
Creating a Hot Spare
Removing Volumes - (Using Solaris 9 Volume Manager Commands)
Removing a State Database Replica
Removing a Stripe - (RAID 0)
Removing a Concatenation - (RAID 0)
Removing a Mirror - (RAID 1)
Removing a RAID5 Volume - (RAID 5)
Removing a Hot Spare

Mirroring Disks with Solstice DiskSuite

• Mirroring Disks with Solstice DiskSuite
• Recovering from a Boot Disk Failure with Solstice DiskSuite
• http://mononoke.mhost.com/~rsletten/unix/disksuite
• Helpful Hints
• Comparisons of Disksuite vs Volume Manager

Using Solaris Volume Manager

Solaris Volume Manager - Soft Partitioning Explained

ITworld.com - Setting Up RAID Volumes with Solaris Volume Manager ...

Managing Disks: Solaris Volume Manager

• RAID 0 Concatenation
• RAID 0 Stripe
• RAID 1 Mirror
• RAID 5
• Soft partitions

Selected BluePrints

Configuring Boot Disks With Solaris Volume Manager Software (October 2002)
-by Erik Vanden Meersch and Kristien Hens
This article is an update to the April 2002 Sun BluePrints OnLine article, Configuring Boot Disks With Solstice DiskSuite Software. This article focuses on the Solaris 9 Operating Environment, Solaris Volume Manager software, and VERITAS Volume Manager 3.2 software. It describe how to partition and mirror the system disk, and how to create and maintain a backup system disk. In addition, this article presents technical arguments for the choices made, and includes detailed runbooks.

Solaris Volume Manager Performance Best Practices (November 2003)
-by Glenn Fawcett
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.

Configuring Boot Disks (December 2001)
-by John S. Howard and David Deeths
This article is the fourth chapter of the Sun BluePrints book titled Boot Disk Management: A Guide For The Solaris Operating Environment (ISBN 0-13-062153-6), which is available through www.sun.com/books, amazon.com, and Barnes & Noble bookstores.

This chapter presents a reference configuration of the root disk and associated disks that emphasizes the value of configuring a system for high availability and high serviceability. This chapter explains the value of creating a system with both of these characteristics, and outlines the methods used to do so.

Wide Thin Disk Striping (October 2000)
-by Bob Larson
In this article, the technique of using stripes to distribute data and indexes over several disks is described. The article also contains the recommendations to use wide-thin stripes to maximize operational flexibility while minimizing complexity.

Reference

DiskSuite 4.2.1 User's Guide

DiskSuite 4.2.1 Reference Guide

docs.sun.com: Solaris Volume Manager Administration Guide

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