Simplified distributed block storage with strong consistency, like in Ceph
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DocumentationConfiguration → Pool configuration


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Pool configuration

Pool configuration is set in etcd key /vitastor/config/pools in the following JSON format:

{
  "<Numeric ID>": {
    "name": "<name>",
    ...other parameters...
  }
}

Pool configuration is also affected by:

Parameters:

Examples:

Placement Tree

OSD placement tree is set in a separate etcd key /vitastor/config/node_placement in the following JSON format:

{ "<node name or OSD number>": { "level": "<level>", "parent": "<parent node name, if any>" }, ... }

Here, if a node name is a number then it is assumed to refer to an OSD. Level of the OSD is always "osd" and cannot be overriden. You may only override parent node of the OSD which is its host by default.

Non-numeric node names refer to other placement tree nodes like hosts, racks, datacenters and so on.

Hosts of all OSDs are auto-created in the tree with level "host" and name equal to the host name reported by a corresponding OSD. You can refer to them without adding them to this JSON tree manually.

Level may be "host", "osd" or refer to some other placement tree level from placement_levels.

Parent node reference is required for intermediate tree nodes.

OSD settings

Separate OSD settings are set in etc keys /vitastor/config/osd/<number> in JSON format {"<key>":<value>}.

As of now, there is only one setting:

reweight

  • Type: number, between 0 and 1
  • Default: 1

Every OSD receives PGs proportional to its size. Reweight is a multiplier for OSD size used during PG distribution.

This means an OSD configured with reweight lower than 1 receives less PGs than it normally would. An OSD with reweight = 0 won't store any data. You can set reweight to 0 to trigger rebalance and remove all data from an OSD.

Pool parameters

name

  • Type: string
  • Required

Pool name.

scheme

  • Type: string
  • Required
  • One of: "replicated", "xor", "ec" or "jerasure"

Redundancy scheme used for data in this pool. "jerasure" is an alias for "ec", both use Reed-Solomon-Vandermonde codes based on ISA-L or jerasure libraries. Fast ISA-L based implementation is used automatically when it's available, slower jerasure version is used otherwise.

pg_size

  • Type: integer
  • Required

Total number of disks for PGs of this pool - i.e., number of replicas for replicated pools and number of data plus parity disks for EC/XOR pools.

parity_chunks

  • Type: integer

Number of parity chunks for EC/XOR pools. For such pools, data will be lost if you lose more than parity_chunks disks at once, so this parameter can be equally described as FTT (number of failures to tolerate).

Required for EC/XOR pools, ignored for replicated pools.

pg_minsize

  • Type: integer
  • Required

Number of available live disks for PGs of this pool to remain active. That is, if it becomes impossible to place PG data on at least (pg_minsize) OSDs, PG is deactivated for both read and write. So you know that a fresh write always goes to at least (pg_minsize) OSDs (disks).

FIXME: pg_minsize behaviour may be changed in the future to only make PGs read-only instead of deactivating them.

pg_count

  • Type: integer
  • Required

Number of PGs for this pool. The value should be big enough for the monitor / LP solver to be able to optimize data placement.

"Enough" is usually around 64-128 PGs per OSD, i.e. you set pg_count for pool to (total OSD count * 100 / pg_size). You can round it to the closest power of 2, because it makes it easier to reduce or increase PG count later by dividing or multiplying it by 2.

In Vitastor, PGs are ephemeral, so you can change pool PG count anytime just by overwriting pool configuration in etcd. Amount of the data affected by rebalance will be smaller if the new PG count is a multiple of the old PG count or vice versa.

failure_domain

  • Type: string
  • Default: host

Failure domain specification. Must be "host" or "osd" or refer to one of the placement tree levels, defined in placement_levels.

Two replicas, or two parts in case of EC/XOR, of the same block of data are never put on OSDs in the same failure domain (for example, on the same host). So failure domain specifies the unit which failure you are protecting yourself from.

max_osd_combinations

  • Type: integer
  • Default: 10000

Vitastor data placement algorithm is based on the LP solver and OSD combinations which are fed to it are generated ramdonly. This parameter specifies the maximum number of combinations to generate when optimising PG placement.

This parameter usually doesn't require to be changed.

pg_stripe_size

  • Type: integer
  • Default: 0

Specifies the stripe size for this pool according to which images are split into different PGs. Stripe size can't be smaller than block_size multiplied by (pg_size - parity_chunks) for EC/XOR pools, or 1 for replicated pools, and the same value is used by default.

This means first pg_stripe_size = (block_size * (pg_size-parity_chunks)) bytes of an image go to one PG, next pg_stripe_size bytes go to another PG and so on.

Usually doesn't require to be changed separately from the block size.

root_node

  • Type: string

Specifies the root node of the OSD tree to restrict this pool OSDs to. Referenced root node must exist in /vitastor/config/node_placement.

osd_tags

  • Type: string or array of strings

Specifies OSD tags to restrict this pool to. If multiple tags are specified, only OSDs having all of these tags will be used for this pool.

primary_affinity_tags

  • Type: string or array of strings

Specifies OSD tags to prefer putting primary OSDs in this pool to. Note that for EC/XOR pools Vitastor always prefers to put primary OSD on one of the OSDs containing a data chunk for a PG.

Examples

Replicated pool

{
  "1": {
    "name":"testpool",
    "scheme":"replicated",
    "pg_size":2,
    "pg_minsize":1,
    "pg_count":256,
    "failure_domain":"host"
  }
}

Erasure-coded pool

{
  "2": {
    "name":"ecpool",
    "scheme":"ec",
    "pg_size":3,
    "parity_chunks":1,
    "pg_minsize":2,
    "pg_count":256,
    "failure_domain":"host"
  }
}