Synnefo » snf-ganeti

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Storage free space reporting

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Background

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Currently Space reporting is broken for all storage types except drbd or

lvm (plain). This design looks at the root causes and proposes a way to

fix it.

Proposed changes

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The changes below will streamline Ganeti to properly support

interaction with different storage types.

Configuration changes

---------------------

Each storage type will have a new "pools" parameter added (type list of

strings). This will be list of vgs for plain and drbd (note that we make

no distinction at this level between allowed vgs and metavgs), the list

of rados pools for rados, or the storage directory for file and

sharedfile. The parameters already present in the cluster config object

will be moved to the storage parameters.

Since currently file and sharedfile only support a single directory this

list will be limited to one. In the future if we'll have support for

more directories, or for per-nodegroup directories this can be changed.

Note that these are just "mechanisms" parameters that define which

storage pools the cluster can use. Further filtering about what's

allowed can go in the ipolicy, but these changes are not covered in this

design doc.

Since the ipolicy currently has a list of enabled storage types, we'll

use that to decide which storage type is the default, and to self-select

it for new instance creations, and reporting.

Enabling/disabling of storage types at ``./configure`` time will be

eventually removed.

RPC changes

-----------

The noded RPC call that reports node storage space will be changed to

accept a list of <method>,<key> string tuples. For each of them it will

report the free amount of space found on storage <key> as known by the

requested method. Methods are for example ``lvm``, ``filesystem``,

``rados``, and the key would be a volume group name in the case of lvm,

a directory name for the filesystem and a rados pool name, for

rados_pool.

Masterd will know (through a constant map) which storage type uses which

method for storage calculation (i.e. ``plain`` and ``drbd`` use ``lvm``,

``file`` and ``sharedfile`` use ``filesystem``, etc) and query the one

needed (or all of the needed ones).

Note that for file and sharedfile the node knows which directories are

allowed and won't allow any other directory to be queried for security

reasons. The actual path still needs to be passed to distinguish the

two, as the method will be the same for both.

These calculations will be implemented in the node storage system

(currently lib/storage.py) but querying will still happen through the

``node info`` call, to avoid requiring an extra RPC each time.

Ganeti reporting

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``gnt-node list`` will by default report information just about the

default storage type. It will be possible to add fields to ask about

other ones, if they're enabled.

``gnt-node info`` will report information about all enabled storage

types, without querying them (just say which ones are supported

according to the cluster configuration).

``gnt-node list-storage`` will change to report information about all

available storage pools in each storage type. An extra flag will be

added to filter by storage pool name (alternatively we can implement

this by allowing to query by a list of ``type:pool`` string tuples to

have a more comprehensive filter).

Allocator changes

-----------------

The iallocator protocol doesn't need to change: since we know which

storage type an instance has, we'll pass only the "free" value for that

storage type to the iallocator, when asking for an allocation to be

made. Note that for DRBD nowadays we ignore the case when vg and metavg

are different, and we only consider the main VG. Fixing this is outside

the scope of this design.

Rebalancing changes

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Hbal will not need changes, as it handles it already. We don't forecast

any changes needed to it.

Space reporting changes

-----------------------

Hspace will by default report by assuming the allocation will happen on

the default storage for the cluster/nodegroup. An option will be added

to manually specify a different storage.

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