Disk Sizing Heavy Write Scenarios

In extreme write scenarios when append-only files (AOF) are enabled, the AOF rewrite process can require considerably more disk space for [database persistence]({{<relref "/operate/rs/7.22/databases/configure/database-persistence/">}}).

For disk size requirements for standard usage, see [Hardware requirements]({{< relref "/operate/rs/7.22/installing-upgrading/install/plan-deployment/hardware-requirements" >}}).

Estimate required disk space

To estimate the required persistent disk space for AOF rewrite purposes in extreme write scenarios, use the following formula:

X (1 + 3Y + Y²)

Where:

• X = size of each shard in GB
• Y = number of shards

Examples

The following examples show how to calculate the persistent disk space required for heavy write scenarios for different database configurations, where:

• Database size is the memory limit configured for the database.

• Number of shards is the total number of shards (primary shards + replica shards).

Example 1

• Database size = 10 GB
• Number of shards = 4

1. Calculate the shard size:

   shCopy

   Shard size = database size / number of shards
              = 10 GB / 4 shards
              = 2.5 GB per shard
   

2. Use the formula to calculate the required persistent disk space:

   shCopy

   Disk space = X (1 + 3Y + Y²)
              = 2.5 (1 + 3 × 4 + 4²)
              = 2.5 (1 + 12 + 16)
              = 2.5 × 29
              = 72.5 GB
   

3. Round up to 73 GB of required disk space.

Example 2

• Database size = 10 GB
• Number of shards = 16

1. Calculate the shard size:

   shCopy

   Shard size = database size / number of shards
              = 10 GB / 16 shards
              = 0.625 GB per shard
   

2. Use the formula to calculate the required persistent disk space:

   shCopy

   Disk space = X (1 + 3Y + Y²)
              = 0.625 (1 + 3 × 16 + 16²)
              = 0.625 (1 + 48 + 256)
              = 0.625 × 305
              = 190.625 GB
   

3. Round up to 191 GB of required disk space.

Example 3

• Database size = 40 GB
• Number of shards = 5

1. Calculate the shard size:

   shCopy

   Shard size = database size / number of shards
              = 40 GB / 5 shards
              = 8 GB per shard
   

2. Use the formula to calculate the required persistent disk space:

   shCopy

   Disk space = X (1 + 3Y + Y²)
              = 8 (1 + 3 × 5 + 5²)
              = 8 (1 + 15 + 25)
              = 8 × 41
              = 328 GB
   

3. Required disk space: 328 GB.

Example 4

• Database size = 40 GB
• Number of shards = 15

1. Calculate the shard size:

   shCopy

   Shard size = database size / number of shards
              = 40 GB / 15 shards
              = 2.67 GB per shard
   

2. Use the formula to calculate the required persistent disk space:

   shCopy

   Disk space = X (1 + 3Y + Y²)
              = 2.67 (1 + 3 × 15 + 15²)
              = 2.67 (1 + 45 + 225)
              = 2.67 × 271
              = 723.57 GB
   

3. Round up to 724 GB of required disk space.