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rbd: document stripe unit options on man page

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Signed-off-by: Sage Weil <sage@inktank.com>
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Sage Weil 2012-10-01 20:57:33 -07:00
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doc/man/8/rbd.rst
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@ -63,6 +63,15 @@ Parameters
Specifies the object size expressed as a number of bits, such that
the object size is ``1 << order``. The default is 22 (4 MB).
.. option:: --stripe-unit size-in-bytes
Specifies the stripe unit size in bytes. See striping section (below) for more details.
.. option:: --stripe-count num
Specifies the number of objects to stripe over before looping back
to the first object. See striping section (below) for more details.
.. option:: --snap snap
Specifies the snapshot name for the specific operation.
@ -108,7 +117,8 @@ Commands
If a snapshot is specified, whether it is protected is shown as well.
:command:`create` [*image-name*]
Will create a new rbd image. You must also specify the size via --size.
Will create a new rbd image. You must also specify the size via --size. The
--stripe-unit and --stripe-count arguments are optional, but must be used together.
:command:`clone` [*parent-snapname*] [*image-name*]
Will create a clone (copy-on-write child) of the parent snapshot.
@ -221,6 +231,35 @@ Thus an image name that contains a slash character ('/') requires specifying the
name explicitly.
Striping
========
RBD images are striped over many objects, which are then stored by the
Ceph distributed object store (RADOS). As a result, read and write
requests for the image are distributed across many nodes in the
cluster, generally preventing any single node from becoming a
bottleneck when individual images get large or busy.
The striping is controlled by three parameters:
.. option:: order
The size of objects we stripe over is a power of two, specifially 2^[*order*] bytes. The default
is 22, or 4 MB.
.. option:: stripe_unit
Each [*stripe_unit*] contiguous bytes are stored adjacently in the same object, before we move on
to the next object.
.. option:: stripe_count
After we write [*stripe_unit*] bytes to [*stripe_count*] objects, we loop back to the initial object
and write another stripe, until the object reaches its maximum size (as specified by [*order*]. At that
point, we move on to the next [*stripe_count*] objects.
By default, [*stripe_unit*] is the same as the object size and [*stripe_count*] is 1. Specifying a different
[*stripe_unit*] requires that the STRIPINGV2 feature be supported (added in Ceph v0.53) and format 2 images be
used.
Examples
========
@ -271,6 +310,10 @@ To create an image and a clone from it::
rbd snap protect mypool/parent@snap
rbd clone mypool/parent@snap otherpool/child
To create an image with a smaller stripe_unit (to better distribute small writes in some workloads)::
rbd -p mypool create myimage --size 102400 --stripe-unit 65536 --stripe-count 16
To change an image from one format to another, export it and then
import it as the desired format::