Concatenated and Stripe

By Suzanne Hart,2014-05-26 18:10
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Concatenated and Stripe


    A simple metadevice is a metadevice built only from slices, and is either used directly or as the basic building block for mirrors and trans metadevices. There are three kinds of simple metadevices: concatenated metadevices, striped metadevices, and concatenated striped metadevices.

    A concatenated metadevice, or concatenation, is a metadevice whose data is organized serially and adjacently across disk slices, forming one logical storage unit. You would use a concatenated metadevice to get more storage capacity by logically combining the capacities of several slices. You can add more slices to the concatenated metadevice as the demand for storage grows.

    A concatenated metadevice enables you to dynamically expand storage capacity and file system sizes online. With a concatenated metadevice you can add slices even if the other slices are currently active.

    Concatenated Metadevice Conventions

    ; When would I create a concatenated metadevice?

    To expand the capacity of an existing data set, such as a file system. Concatenation is good for small random I/O and for even I/O distribution. ; What are the limitations to concatenation?

    Practically speaking, none. You must use a concatenation to encapsulate root (/), swap, /usr, /opt, or /var when mirroring these file systems.

    ; How large can a concatenated metadevice be?

    Up to one Terabyte.

Striped Metadevice (Stripe)

    A striped metadevice, or stripe, is a metadevice that arranges data across two or more

    slices. Striping alternates equally-sized segments of data across two or more slices, forming one logical storage unit. These segments are interleaved round-robin, so that the combined space is made alternately from each slice, in effect, shuffled like a deck

    of cards.

    Note - Sometimes a striped metadevice is called a “stripe.” Other times, “stripe”

    refers to the component blocks of a striped concatenation. “To stripe” means to

    spread I/O requests across disks by chunking parts of the disks and mapping those chunks to a virtual device (a metadevice). Striping is also classified as RAID level 0,

    as is concatenation.

    While striping and concatenation both are methods of distributing data across disk slices, striping alternates chunks of data across disk slices, while concatenation distributes data “end-to-end” across disk slices.

    For sequential I/O operations on a concatenated metadevice, DiskSuite reads all the

    blocks on the first slice, then all the blocks of the second slice, and so forth. For sequential I/O operations on a striped metadevice, DiskSuite reads all the blocks in a segment of blocks (called an interlace) on the first slice, then all the blocks in a

    segment of blocks on the second slice, and so forth.

    On both a concatenation and a striped metadevice, all I/O occurs in parallel.

Striped Metadevice Conventions

    ; Why would I create a striped metadevice?

    To take advantage of the performance increases that come from accessing data in parallel and to increase capacity. Always use striped metadevices for new file systems or data sets.

    Striping enables multiple controllers to access data at the same time (parallel access). Parallel access can increase I/O throughput because all disks in the metadevice are busy most of the time servicing I/O requests.

    Striping is good for large sequential I/O and for uneven I/O.

    ; What are the limitations to striping?

    An existing file system cannot be directly converted to a striped metadevice. If you need to place a file system on a striped metadevice, you can back up the file system, create a striped metadevice, then restore the file system to the striped metadevice.

    When creating a stripe, do not use slices of unequal size, as this will result in unused disk space.

What is an interlace value?

    The size, in Kbytes, Mbytes, or blocks, of the logical data chunks in a striped metadevice. Depending on the application, different interlace values can increase performance for your configuration. The performance increase comes from several disk arms doing I/O. When the I/O request is larger than the interlace size, you may get better performance.

    ? What is DiskSuite’s default interlace value?

    16 Kbytes.

    ; Can I set the interlace value?

    Yes, when you create a new striped metadevice, using either the command line or DiskSuite Tool. Once you have created the striped metadevice, you cannot change the interlace value.

    ; Can I set the interlace value on an existing striped metadevice? No. (Though you could back up the data on it, delete the striped metadevice, create a new striped metadevice with a new interlace value, and then restore the data.)

Concatenated Stripe Conventions

    ; Why would I use a concatenated stripe?

    This is the only way to expand an existing striped metadevice.

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