Dell PowerFlex 4.5.x Technical Overview

SDR journal capacity

It is important to assign enough storage capacity for the replication journal. You should consider the following factors when allocating journal capacity.

• The storage pools from which to allocate the journal capacity. The journal is shared between all of the replicated RCGs in the protection domain. Journal capacity should be allocated from storage pools as fast as (or faster than) the storage pool of the fastest replicated application in the protection domain. It should use the same drive technology and about the same drive count and distribution in nodes.
• The minimal requirements needed, for which the default minimum journal size is 400 GB. The minimum journal size can also be calculated as 28 GB per SDR session (where the number of SDR sessions is the number of SDRs + 1) or the capacity needed to sustain an outage, whichever is greater.
• Expected outage time.
  • The minimal outage allowance is one hour, but at least three hours are recommended when computing the minimum size of the replication journal.
  • Journal capacity needed per application is: maximal application throughput x maximum outage interval.
• Journal capacity as calculated as a percentage of storage pool capacity, based on the previously calculated needs. Journal capacity should be at least 5% of replicated usable capacity in the protection domain, including volumes used as source and targets.

For example:

• An application generates 1 GB/s of writes.
• The maximal supported outage is 3 hours (3 hours x 3600 seconds = 10800 seconds).
• The journal capacity needed for this application is 1 GB/s x 10800 s = ~10.547 TB.
• Since the journal capacity is expressed as a percentage of the storage pool capacity, divide the 10.547 TB by the size of the storage pool, which is 200 TB: 100 x 10.547TB/200TB = 5.27% Round this up to 6%.
• Repeat this for each application being replicated.

When a protection domain has several storage pools and several replicated applications, the journal capacity should be calculated as in the example above, and the capacity can be divided among all the storage pools (provided they are fast enough). For higher availability, the journal capacity should be allocated from multiple storage pools.

It is important to note that since journal capacity is defined as a percentage of the total storage capacity in the storage pool, increasing the total storage capacity by adding devices will increase the journal capacity. Similarly, if you decrease the total storage capacity by removing devices from the storage pool, the journal capacity will automatically decrease.