Our product offers near real time (incremental sync every 3 mins) backup and restore solution for on-premise SQLServer data. We have used PostgreSQL to store the backup data in cloud and offered add-on services on top of PostgreSQL data (such as reports, analytics, etc). Every customer data is stored as a separate database.

Initially we have used RDS as PostgreSQL instance when the product is in pilot phase. RDS costed us $548 for just 2vCPU 16GB 500GB SSD (db.r5.large Multi-AZ). Considering the increasing active customer base and volume of data involved, we found that RDS is very expensive and costed us more (> 100%) than the market affordable estimated product pricing (:facepalm:). As per our performance benchmark, we found that db.r5.large can accommodate 250 customers and scalable linearly. To reduce the RDS spend, we had to reduce two costs.

1) Reduce the RDS Instance cost / customer - we aggressively optimised our sync flow and final benchmark reveals 500 customers can be accommodated in db.r5.large Instance (50% less RDS Instance spend / customer)

2) Reduce the RDS Storage cost / GB of customer data - we could not find any way to reduce the storage cost. Since RDS Instance is fully managed by AWS, no possibility of data compression.

When we compared the total cost based on our usage, RDS Instance cost is just 10-20% and Storage cost is 80-90%. So finally we decided to host our PostgreSQL Instance in EC2 with transparent data compression. This is our current configuration and usage metrics.

r5a.xlarge (4vCPU 32GB)

PG Master - Availabilty Zone 1

PG Slave - Availabilty Zone 2 (Streaming Replication)

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8 x 100GB ZFS RAID0 FileSystem with LZ4 Compression (128KB recordsize)

40GB (wal files) ZFS FileSystem with LZ4 Compression (1MB recordsize)

600GB Compressed Data (3.1TB Uncompressed - x5.18 compression ratio)

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2 x r5a.xlarge - 2 x $104.68 = $209.36

2 x 8 x 100GB - 2 x 8 x $11.40 = $182.40

2 x 40GB - 2 x $4.56 = $9.12

Total EC2 Cost = $400.88

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If we had to use RDS

db.r5.xlarge Multi-AZ = $834.48

3.5TB Multi-AZ = $917.00

Total RDS Cost = $1751.48

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So we have reduced our cost by $1751.48 / month (greater than x4 times) by using EC2 instead of RDS. Best of all we have purchased 3 Years No Upfront Savings Plan which further reduced our EC2 Instance cost to $105 (50% reduction). RDS doesn't have No Upfront Reserved plan for 3 Years and for 1 Year No Upfront we get just 32% Instance cost reduction.

Apart from the direct EC2 Instance cost and Storage size reduction, major benefit we indirectly got by migrating to EC2 Instances is

- IO Throughput increased by x5 due to ZFS LZ4 Compression. Importing of 3GB Compressed GZIP file would take around 2.5 - 3 hrs in RDS whereas in EC2 it just takes 30 - 45 min.

- Existing Savings Plan discount automatically applied (50% reduction)

- Ability to migrate to AMD based Instances (r5a.xlarge) - 50% reduction compared to Intel based Instances (r5.xlarge) in Mumbai region. It'll take ages before AMD based Instances are available in RDS.

- Ridiculous EBS Burst Credits by using 8 Volumes in RAID0. Base IOPS - 8 x 300 (100GB) = 2400 IOPS. Burst IOPS Credits - 8 x 3000 = 24000 IOPS :D

- PG Master is used for backup sync write operations and PG Slave is used for reporting and analytics. RDS requires Read Replica to be created from the already existing db.r5.xlarge Multi-AZ Instance for read operation which will further increase the estimated RDS cost by x1.5

- Planning to migrate to AWS Graviton 2 ARM64 Instances. AMD (r5a.xlarge) and Intel (r5.xlarge) based Instances have hyperthreading enabled which leaves us with just 2 real cores and 4 threads. But basic Graviton 2 Instance r6g.large itself has 2 real cores. So I'm kinda estimating that the basic r6g.large (2vCPU 16GB) Instance itself can support upto 1000 Active Customers (further 50% EC2 Instance Cost reduction).