Concatenated and Stripe

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串联与条带
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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