openstack/openstack-manuals
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Support for ext4 as default for ephemeral disks

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Currently, ext4 is supported as default for all Linux.
Also, for non-linux and non-windows OSs the default is vfat
(used to be ext3).

There is still support for ext3 as the default if some
deployers do choose to keep that as the default, by using either
virt_mkfs or default_ephemeral_format configuration option.

Change-Id: I1b315043d0738bc4eec6739ad82be52e395f8d58
Closes-Bug: #1415407
This commit is contained in:
OlgaGusarenko 2015-02-02 15:10:40 +02:00
parent 9b3f4661dd
commit a0c12a5f05
1 changed files with 67 additions and 47 deletions
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114
doc/admin-guide-cloud/ch_compute.xml
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@ -182,66 +182,86 @@
</section>
<section xml:id="section_storage-and-openstack-compute">
<title>Block storage</title>
<para>OpenStack provides two classes of block storage:
ephemeral storage and persistent volumes. Volumes are
persistent virtualized block devices independent of any
particular instance.</para>
<para>Ephemeral storage is associated with a single unique
instance, and it exists only for the life of that
instance. The amount of ephemeral storage is defined by
the flavor of the instance. Generally, the root file
system for an instance will be stored on ephemeral
storage. It persists across reboots of the guest operating
system, but when the instance is deleted, the ephemeral
storage is also removed.</para>
<para>In addition to the ephemeral root volume, all flavors
except the smallest, <filename>m1.tiny</filename>, also
provide an additional ephemeral block device of between 20
and 160&nbsp;GB. These sizes can be configured to suit your
environment. This is presented as a raw block device with
no partition table or file system. Cloud-aware operating
system images can discover, format, and mount these
storage devices. For example, the <systemitem
class="service">cloud-init</systemitem> package
included in Ubuntu's stock cloud images format this space
as an <filename>ext3</filename> file system and mount it
on <filename>/mnt</filename>. This is a feature of the
guest operating system you are using, and is not an
OpenStack mechanism. OpenStack only provisions the raw
storage.</para>
<para>Persistent volumes are created by users and their size
is limited only by the user's quota and availability
limits. Upon initial creation, volumes are raw block
devices without a partition table or a file system. To
partition or format volumes, you must attach them to an
instance. Once they are attached to an instance, you can
use persistent volumes in much the same way as you would
use external hard disk drive. You can attach volumes to
only one instance at a time, although you can detach and
reattach volumes to as many different instances as you
like.</para>
<para>You can configure persistent volumes as bootable and use
them to provide a persistent virtual instance similar to
traditional non-cloud-based virtualization systems.
It is still possible for the resulting instance to also have
<para>OpenStack provides two classes of the block storage:
ephemeral storage and persistent volume.</para>
<simplesect>
<title>Ephemeral storage</title>
<para>An ephemeral storage includes a root ephemeral volume
and an additional ephemeral volume.</para>
<para>The root disk is associated with an instance,
and exists only for the life of this very
instance. Generally, it is used
to store an instance`s root file system, persists across
the guest operating system reboots, and is removed
on an instance deletion. The amount of the root ephemeral
volume is defined by the flavor of an instance.</para>
<para>In addition to the ephemeral root volume, all default types of flavors,
except <literal>m1.tiny</literal>, which is
the smallest one, provide an additional ephemeral block
device sized between 20 and 160&nbsp;GB (a configurable
value to suit an environment).
It is represented as a raw block device with
no partition table or file system. A cloud-aware operating
system can discover, format, and mount such a
storage device. OpenStack Compute defines the default file system for
different operating systems as Ext4 for Linux distributions,
VFAT for non-Linux and non-Windows operating systems, and
NTFS for Windows. However, it is possible to specify
any other filesystem type by using <parameter>virt_mkfs</parameter> or
<parameter>default_ephemeral_format</parameter> configuration options.</para>
<note>
<para>For example, the <systemitem class="service">cloud-init</systemitem> package
included into an Ubuntu's stock cloud image, by default,
formats this space as an Ext4 file system and mounts
it on <filename>/mnt</filename>.
This is a cloud-init feature, and is not an OpenStack mechanism.
OpenStack only provisions the raw storage.</para>
</note>
</simplesect>
<simplesect>
<title>Persistent volume</title>
<para>A persistent volume is represented by a persistent virtualized block device
independent of any particular instance, and provided by OpenStack Block Storage.</para>
<para>A persistent volume is created by a user, and its size
is limited only by a user's quota and availability
limits. Upon initial creation, a volume is a raw block
device without a partition table or a file system. To
partition or format a volume, you must attach it to an
instance. Once it is attached, it can be used
the same way as an external hard disk drive.
A single volume can be attached to one instance at a time,
though you can detach and reattach it to other instances
as many times as required.</para>
<para>You can configure a persistent volume as bootable and use
it to provide a persistent virtual instance similar to
the traditional non-cloud-based virtualization system.
It is still possible for the resulting instance to keep
ephemeral storage, depending on the flavor selected. In this
case, the root file system can be on the persistent volume
and its state maintained even if the instance is shut down.
case, the root file system can be on the persistent volume,
and its state is maintained, even if the instance is shut down.
For more information about this type of configuration, see
the <link
xlink:href="http://docs.openstack.org/juno/config-reference/content/">
<citetitle>OpenStack Configuration Reference</citetitle></link>.
</para>
<note>
<para>Persistent volumes do not provide concurrent access
<para>A persistent volume does not provide concurrent access
from multiple instances. That type of configuration
requires a traditional network file system like NFS or
requires a traditional network file system like NFS, or
CIFS, or a cluster file system such as GlusterFS.
These systems can be built within an OpenStack cluster
These systems can be built within an OpenStack cluster,
or provisioned outside of it, but OpenStack software
does not provide these features.</para>
</note>
</simplesect>
</section>
<section xml:id="instance-mgmt-ec2compat">
<title>EC2 compatibility API</title>
<para>In addition to the native compute API, OpenStack provides