> ## Documentation Index
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> System table containing information about storage policies and volumes which are defined in server configuration.

# system.storage_policies

<h2 id="description">
  Description
</h2>

Contains information about storage policies and volumes which are defined in [server configuration](/reference/engines/table-engines/mergetree-family/mergetree#table_engine-mergetree-multiple-volumes_configure).

<h2 id="columns">
  Columns
</h2>

* `policy_name` ([String](/reference/data-types)) — The name of the storage policy.
* `volume_name` ([String](/reference/data-types)) — The name of the volume.
* `volume_priority` ([UInt64](/reference/data-types)) — The priority of the volume.
* `disks` ([Array(String)](/reference/data-types)) — The list of all disks names which are a part of this storage policy.
* `volume_type` ([Enum8('JBOD' = 0, 'SINGLE\_DISK' = 1, 'UNKNOWN' = 2)](/reference/data-types)) — The type of the volume - JBOD or a single disk.
* `max_data_part_size` ([UInt64](/reference/data-types)) — the maximum size of a part that can be stored on any of the volumes disks.
* `move_factor` ([Float32](/reference/data-types)) — When the amount of available space gets lower than this factor, data automatically starts to move on the next volume if any (by default, 0.1).
* `prefer_not_to_merge` ([UInt8](/reference/data-types)) — You should not use this setting. Disables merging of data parts on this volume (this is harmful and leads to performance degradation).
* `perform_ttl_move_on_insert` ([UInt8](/reference/data-types)) — Disables TTL move on data part INSERT. By default (if enabled) if we insert a data part that already expired by the TTL move rule it immediately goes to a volume/disk declared in move rule.
* `load_balancing` ([Enum8('ROUND\_ROBIN' = 0, 'LEAST\_USED' = 1)](/reference/data-types)) — Policy for disk balancing, `round_robin` or `least_used`.

<h2 id="volume-selection-on-insert">
  Volume selection on `INSERT`
</h2>

When `INSERT` creates a new data part, ClickHouse picks a destination disk
by trying the rules below in order. The first rule that matches **and can
reserve space for the part** wins; otherwise (rule does not apply, no free
space, or `max_data_part_size` exceeded) evaluation continues with the next
rule.

1. **TTL move rule** — if a `TTL <expr> TO VOLUME 'X'` (or `TO DISK 'X'`)
   clause is already in the past for the rows being inserted, **and**
   `perform_ttl_move_on_insert = 1` (default) on the **TTL destination
   volume** (for `TO DISK 'X'`, the volume containing disk `X`), the part
   is written directly to that destination. If reservation there fails, the
   insert falls back to steps 2–4; a warning is logged but the `INSERT`
   does not fail for this reason alone.
2. **`max_data_part_size`** — a volume rejects parts larger than its
   `max_data_part_size`. This is checked per volume; it does not gate a
   step-1 `TTL ... TO DISK 'X'` reservation, which targets the disk
   directly.
3. **`volume_priority`** — among the remaining volumes, the one with the
   lowest `volume_priority` value is chosen. Volumes without an explicit
   `<volume_priority>` are ordered by their position in the configuration.
4. **`load_balancing`** — once a volume is chosen, the disk inside that
   volume is selected according to its `load_balancing` policy
   (`round_robin` or `least_used`).

<Info>
  **Override**

  If `min_free_disk_bytes_to_perform_insert` or
  `min_free_disk_ratio_to_perform_insert` is non-zero, the precedence above
  is bypassed. `INSERT` tries only the volume with the lowest
  `volume_priority` and throws `NOT_ENOUGH_SPACE` if no disk in that volume
  meets the threshold. Inserts into the `system` database are exempt.
</Info>

<Note>
  `perform_ttl_move_on_insert` is read from the **TTL destination** volume,
  not from the source volume. For a `TO DISK 'X'` rule, the flag is read
  from the volume that contains disk `X`. Setting it on any other volume of
  the policy has no effect on the insert path.
</Note>

To force inserts to honour `volume_priority` even when an "already
expired" TTL move rule applies, set `perform_ttl_move_on_insert = 0` on
the TTL destination volume (for `TO DISK 'X'`, on the volume that contains
disk `X`). The part is then written to the priority-N volume first and
moved to the TTL destination by a background move task (observable via
`system.moves`). See the
[`perform_ttl_move_on_insert` setting on the MergeTree engine](/reference/engines/table-engines/mergetree-family/mergetree#table_engine-mergetree-multiple-volumes_configure).