Managing Storage Volume

manager (RAID & SVM)

Study Purpose Only

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Advantage of Storage Vol. Mgr.

1. The availability of abundant disk space makes it possible to provide data replication, thereby implementing fault tolerance.

2. Because a volume can span across multiple disks, we can write to multiple disks simultaneously to improve performance.

• two features are implemented in a technology called

Redundant Array of Inexpensive/Independent Disks (RAID).

Solaris Volume Manager (SVM) [Solaris tool ]

• disk set is takes the management of disk space from a single disk to multiple disks.

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Disk Mirroring

• Disk mirroring is a process of designating a disk drive, say disk B, as a duplicate (mirror) of another disk drive, say disk A.

• mirroring has a single point of failure—that is, if the disk controller fails, both disks will become inaccessible. The solution to this problem is disk duplexing.

Disk Duplexing

• Disk duplexing solves this problem by providing each disk drive its own disk controller.

• duplexing improves fault tolerance over mirroring.

Disk Striping • Disk striping breaks the data into small pieces called stripes,

and those stripes are written to multiple disks simultaneously.

• read/write heads are working simultaneously, striping improves read/write performance.

Data Redundancy by Parity

• If you lose a part of data, you can reconstruct it from the parity information of this data.

• When a chunk (say a block) of data is stored on a disk, its parity information is calculated and stored on another disk. If this part of the data is lost, it can be reconstructed from the parity information.

• Note that parity provides fault tolerance at the cost of performance and disk space, because parity calculations take CPU time, and parity information takes disk space.

RAID • Redundant Array of Inexpensive/Independent Disks

• Uses a set of redundant disks to provide fault tolerance.

• Disks usually reside together in a cabinet and are therefore referred to as an array.

RAID levels:

RAID 0:

• This level of RAID uses striping on multiple disks.

• Means that a volume contains stripes on multiple disks and the data can be written simultaneously to these stripes.

• RAID 0 improves performance

• Does not implement fault tolerance even though it uses multiple disks

• If one disk fails that contains a stripe of the file, you lose the file.

RAID 1:

• RAID level uses mirroring (or duplexing) on two disks to provide a very basic level of disk fault tolerance

• Both disks contain the same data—that is, one disk is a mirror image of the other.

• also provides performance improvement in data reads, because if one disk is busy, the data can be read from the other disk.

RAID 1 + 0:

• mirroring is striped to provide performance improvement in addition to fault tolerance.

RAID 2:

• This RAID level implements striping with parity

• Needs at least three disks because parity information is written on a disk other than the data disks.

• The striping is done at bit level—that is, bits are striped across multiple disks.

RAID 3:

• Similar to RAID 2 (striping with parity) the striping is done at byte level as opposed to bit level

• Offers improved performance over RAID 2, because more data can be read or written in a single read/write operation.

RAID 4:

• Similar to RAID 2 and RAID 3 (striping with parity).

• Main difference is that the striping is done at block level as opposed to bit level or byte level

• Offers improved performance because more data can be read or written in a single read/write operation.

RAID 5:

• Implements striping with distributed parity

• That is, parity is also striped across multiple disks

• The data and parity can be interleaved on all disks

• Parity of a given piece of data on one disk must not be written to the same disk, which would defeat the whole purpose of parity.

• RAID 5 requires three disks at minimum

SVM (Solaris Volume Manager)

• A volume is a named chunk of disk space that can occupy a part of a disk or the whole disk

• SVM is a software product that helps you manage huge amounts of data spread over a large number of disks.

■ Increasing storage capacity

■ Increasing data availability

■ Easing administration of large storage devices

important concepts related to SVM:

Soft partition:

• Divide a disk slice or a logical volume into as many divisions as needed.

• Maximum size of a soft partition is limited to the size of the slice or the logical volume of which it is a part.

Hot spare :

• Hot spare is a slice that is reserved for automatic substitution in case the corresponding data slice fails.

• Using SVM, you can dynamically add, delete, replace, and enable hot spares within the hot spare pool.

Hot spare pool:

• Hot spare pool is a collection (an ordered list) of hot spares that SVM uses to provide increased data availability and fault tolerance for a RAID 1 (mirrored) volume and a RAID 5 (striped data with striped parity) volume.

Disk set:

• Is a set of disk drives that contain logical volumes and hot spare pools.

State database:

• State database contains the configuration and status information about the disk sets, and about volumes and hot spares in a disk set managed by SVM.

• SVM also maintains replicas (multiple copies) of a state database to provide fault tolerance.

Logical Volumes Supported by SVM

• Logical volume is a group of physical disk slices that appears to be a single logical device.

Logical volumes are used to

increase storage capacity,

data availability (and hence fault tolerance),

and performance (possibly including I /O performance).

RAID 0 Volume

• Volumes enable you to dynamically expand disk storage capacity. • Three kinds of RAID 0 volumes

Stripe volume :

• Volume that spreads data across two or more components (slices or soft partitions)

• Size of the data segment called the interlace size (16KB)

Concatenation volumes:

• Writes data on multiple components sequentially. That is, it writes the data to the first available component until it is full; then it moves to write to the next component.

• No parallel access.

Concatenated stripe volume:

• Volume is a stripe volume that has been expanded by adding components

RAID 1 Volumes

• Each copy of a RAID 0 volume in a RAID 1 volume (mirror) is called a submirror.

• For a volume to be a mirror volume it must contain at least two submirrors, but SVM supports up to four submirrors in a RAID 1 volume.

• The same data will be written to more than one submirror

• Rise to multiple read/write options for a RAID 1 volume.

RAID 5 Volumes

RAID 5 volume is a striped volume that also provides fault tolerance by using parity information distributed across all components

• When we lose data we must not lose its parity information (data and its parity must not reside on the same component)

• A component that already contains a file system must not be included in the creation of a RAID 5 volume, because doing so will erase the data during initialization.

• Use components of equal size.

• Set the interlace value (size of data segments); otherwise, it will be set to a default of 16KB.

RAID 5 uses parity, which slows down performance, it may not be a good solution for write intensive applications.

Configuration

In command mode

• “metadb” command to create, delete, or check the status of a state database.

Creating State Databases

• Default size for a state database replica in SVM is 8192 blocks (4MB)

Example:

• To create an initial (first) state database replica on a new system c0t1d0s7

• # metadb -a -f c0t1d0s7

metadb command

metadb []

values for :

-a. Add a database replica. Used with the -f option when no database replica exists.

-d. Delete all replicas located on the specified slice and update the /kernel/drv/md.conf and /etc/lvm/mddb.cf files accordingly.

-f. Use with -a to create the database when no replica exists.

-i. Inquire about the status of the replica.

value of :

-c . Specify the number of replicas to be added to a specified size. The default is 1.

-l . Size in blocks of the replica that is to be created. The default is 8192.

Performing Mirroring and Unmirroring

Building a Mirror:

Step I : Identify the slice that contains the existing file system to be mirrored, Take example slice c0t0d0s0.

Step II : Create a new RAID 0 volume on the slice from step 1. metainit -f

• use the -f option when the slice contains a mounted fi le system.

■ . Specify the name of the volume that you are creating.

■ . Specify the number of stripes to create in thevolume.

■ . Specify the number of components each

stripe should have.

■ . Specify the names of the components that will

be used to create the volume, c0t0d0s0 in this example.

Mounted FS -> reboot need

Unmounted FS -> reboot not necessary

STEP III : Create a second RAID 0 volume on an unused slice,

Example : c1t1d0s0, to act as the second submirror.

Step IV : Create a one-way mirror

metainit -m

-m option means create a mirror

-> specifies the name of the volume that you want to create

argument specifies the name of the component that will be the first submirror in the mirror.

Step V:

File system you are mirroring is not the root(/) file system, then edit the /etc/vfstab file to make sure that the file system mount instructions refer to the mirror, not to the block device.

For example, an entry like the following in the /etc/vfstab file would be wrong:

• /dev/dsk/ /dev/rdsk/ /var ufs 2 yes –

Change this entry to read:

• /dev/md/dsk/ /dev/md/rdsk/ /var ufs 2 yes -

• Step VI: Remount your newly mirrored file system

■ If you are mirroring the root(/) file system, execute the metaroot command to tell the system to boot from the mirror, and then reboot.

• # metaroot

• # reboot

■ If you are mirroring a file system that is not the root(/) and that cannot be unmounted, just reboot your system.

• If you are mirroring a file system that can be unmounted, you do not need to reboot. Just unmount and remount the file system:

• # umount

• # mount

Step VII: Attach the second submirror by issuing the metattach command:

metattach

Unmirroring a Mounted File System

Step 1:

Become superuser.

Step 2:

Issue the following command to verify that at least one submirror is in the Okay state.

• # metastat

Step 3:

Issue the following command to detach the submirror that you want to continue using for the file system:

• # metadetach

Step 4:

Use one of the following commands depending on the filesystem you want to unmirror:

■ For the root(/) file system, execute the metaroot command to tell the system where to boot from:

• # metaroot

■ For the /usr, /opt, or swap file systems, change the file system entry in the /etc/vfstab fi le to use a non-SVM device (slice).

Step 5:

Reboot the system:

• # reboot

Step 6:

Clear the remaining mirror and submirrors:

• # metaclear -r