PowerFlex Whitepapers Flashcards
What is the role of the SDR?
to proxy the IO of replicated volumes between the SDC and the SDSs where data is ultimately stored
How does the SDR work?
write IO operations are split - sending one copy to the SDS and another copy to the replication journal volume
Where is the SDR in the architecture?
sits between the SDC and SDS and is deployed alongside the SDS nodes
What does the SDR appear to be from the SDS point of view?
like an SDC sending writes
What does the SDR appear to be from the SDC point of view?
like an SDS to which writes can be sent
What method does PowerFlex utilize instead of snapshotting?
journaling
What is a limitation of snapshot based replication?
identifying block change delta is easy but as RPOs get smaller the number of required snapshots increases dramatically
places hard limit on how small RPOs can be
What is the advantage of journal based replication?
provides possibility of smallest RPO and not constrained by the maximum number of snapshots available in a system/volume
How are journals maintained in PowerFlex?
live as volumes in an SP in the same PD
journal volume does not need to reside in the same SP as the volume being replicated
What is important to know about sizing journal volumes for replication?
journal volume must have enough available capacity to continue ingesting replication data even when the WAN is down and target site is not available to send
must consider the maximum cumulative writes that might occur in an outage
What is the minimum requirement for journal capacity?
28GB x # of SDR sessions
SDR sessions = # of SDRs installed + 1
reserve at least 5% of SP for journal volumes
How can reserve journal capacity be distributed?
can be split into several volumes across multiple SPs or can reside all in one SP in a PD
What is the performance requirement for journal volumes?
performance of any SP where journal volume resides must match or exceed performance of SP where replicated volumes reside
What is the single most important consideration when sizing journal capacity?
possible WAN outage
How would you assess the journal capacity needed per application?
need to know the maximum application write bandwidth during the busiest hour
minimum outage allowance is 1hr - strongly recommend using 3 hr allowance
What is an example of journal capacity calculation per application?
Calculation example:
Our application generates 1 GB of writes during peak hours.
Using 3 hours as the supported outage, we calculate from 10,800 seconds.
The journal capacity reservation needed is 1 GB/s * 10800 s = ~10.547 TB.
Because journal capacity is calculated as a percentage of storage pool capacity, we divide the needed space by the storage pool usable capacity. Let us assume that usable capacity is 200 TB.
100 * 10.547 TB / 200 TB = 5.27%.
As a safety margin, we will round up to 6%.
Repeat the calculation for each application being replicated.
How does the SDR organize data as it’s being written?
assembles journal files that contain checkpoints to preserve write order
What happens to duplicate blocks that get sent to the journal volume?
consolidated to minimize volume of data being sent over
How is data transferred from source to target on PowerFlex?
SDR sends data over dedicated local subnets or external WAN networks assigned to replication
How is compression affected in replicated volumes?
compressed data is not sent over the WAN
SDS responsible for compressing writes
What is a volume migration limitation related to replication in PowerFlex?
migrating replicated volumes from one PD to another is not possible
since replication journals don’t span PDs
What are the asynchronous replication topologies available on PowerFlex?
one directional
bi-directional
one-to-many
many-to-one