Understanding Amazon DynamoDB billing for backups
This guide provides details about how DynamoDB billing works for backups. We'll break down the various components that contribute to the overall cost, providing clear explanations and practical examples.
DynamoDB offers on-demand backups and point-in-time recovery (PITR) backups to help protect your DynamoDB data from disaster events and offers data archiving for long-term retention.
How it works
DynamoDB on-demand backups are billed monthly. If you take a backup on any particular day of the month, you’ll see a single charge for that backup calculated for the remaining days of the month (example: creating a backup on the 27th, you will only be charged for the few days remaining in that month, applied as a single charge on the 27th).
If you retain your previously taken backups for subsequent months, you'll always see a full month’s charge for that backup applied on the 1st. If the backup is removed before the month’s end, the charges will be adjusted based on actual usage.
As an example, if you created a backup on July 27th, and it is maintained through the month of August, you will see the following charges for that backup:
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A charge on July 27th for the remaining days of July
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A charge on August 1st for the entire month of August
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A charge on the 1st of every subsequent month that backup exists
-
If the backup is deleted on the 15th of the following month, the charges for that backup will be adjusted down to only the 15 days it was present, still applied on the 1st
When backups are maintained for DynamoDB tables, you may observe that the expense for the
DynamoDB (Region)-TimedBackupStorage-ByteHrs usage metric seems abnormally
high on the 1st of the month. In addition, if you check this metric at the start of a
new month and compare it against previous billing cycles, you may observe what appears
to be a large spike in usage. This is by design. On the 1st of every month, any existing
DynamoDB backups will have usage charges for the entire month applied. Any DynamoDB backups
that are removed during the month will have their usage expense prorated to reflect
actual usage. As a result, you may see the charge (applied on the 1st) decrease
throughout the month. This is because retention policies apply expirations or manual
deletions to carried over backups occur. This will be explored in a scenario below.
DynamoDB backup billing example
Here is an example of what you may see in Cost Explorer at the start of the month:
Notice how February 1st appears to have a much larger spike relative to previous months. Let’s break down why this occurs.
From the DynamoDB Pricing
Page
“The total backup storage size billed each month is the sum of all backups of DynamoDB tables. DynamoDB monitors the size of on-demand backups continuously throughout the month to determine your backup charges.”
This explains why the bill consistently shows a large spike in usage on the 1st of every month. Any existing backups coming into a new month have a full month’s charges applied to the 1st. Put another way, if you enter the month with 300 DynamoDB backups, you will see a full month’s usage charges applied on the 1st day of the month for all 300 backups.
In contrast, any new backups taken throughout the month will show a spike in charge for that backup on the day it is taken, as it is charging for the remainder of the month.
Why does the current month’s usage appear to be so much higher on the 1st than previous months, and what happens if I remove the backups?
To answer this important 2-part question, let’s set up an example scenario using the following information:
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Length of Month: 30 Days
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DynamoDB Backup Frequency: 10/day, 300/mo
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DynamoDB Backup Retention Policy: 30 Days
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DynamoDB Per-Backup Cost: $2/day, $60/mo
-
Previous 1st of Month Total (
TimedBackupStorage-ByteHrs, checked on the 1st of the Current Month): $9,300 -
Previous Month Total (
TimedBackupStorage-ByteHrs): $18,600 -
Current 1st of Month Total (
TimedBackupStorage-ByteHrs, Checked on the 1st): $18,000 -
Changes in DynamoDB Usage Month-to-Month: None
Using the information above, we can see that 300 backups were created in the previous month with a policy to maintain them for 30 days. On the 1st of a new month, all of these backups still remain as they've not yet hit the end of their retention period. However, with each passing day, the oldest sets of backups will begin to drop off, as shown here:
| New month | Day 1 | Day 2 | Day 3 | Day 4 | Day 5 |
|---|---|---|---|---|---|
| Total previous month backups carried over | 300 | 290 | 280 | 270 | 260 |
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On the 1st, we can see 300 backups @ $60/mo per backup, totaling $18,000 of
TimedBackupStorage-ByteHrsapplied. This is in contrast to the previous month, where the entire month’s total was $18,600. -
On the 2nd, 10 of those backups will have expired and drop off. When this occurs, the applied charge for those backups will be adjusted to Actual Usage instead of Assumed Usage. This results in those 10 backups, previously with an applied charge on the 1st of $600 (10 Backups x 30 Days) being adjusted down to $20 (10 Backups x 1 Day).
-
The following day, the next block of 10 will expire and drop, shifting their usage from 30 days down to 2 days, reducing their charge to $40 (10 Backups x 2 Days).
With every passing day, we’ll see that larger-than-previous-month spike begin to shrink. If we expand this to cover the entire month, we’ll observe the following:
| 300 backups in blocks of 10 | 1st | 10th | 20th | 30th |
|---|---|---|---|---|
| Block 1 | $600 | $20 | $20 | $20 |
| Block 2 | $600 | $40 | $40 | $40 |
| Block 3 | $600 | $60 | $60 | $60 |
| Block 4 | $600 | $80 | $80 | $80 |
| Block 5 | $600 | $100 | $100 | $100 |
| Block 6 | $600 | $120 | $120 | $120 |
| Block 7 | $600 | $140 | $140 | $140 |
| Block 8 | $600 | $160 | $160 | $160 |
| Block 9 | $600 | $180 | $180 | $180 |
| Block 10 | $600 | $600 | $200 | $200 |
| Block 11 | $600 | $600 | $220 | $220 |
| Block 12 | $600 | $600 | $240 | $240 |
| Block 13 | $600 | $600 | $260 | $260 |
| Block 14 | $600 | $600 | $280 | $280 |
| Block 15 | $600 | $600 | $300 | $300 |
| Block 16 | $600 | $600 | $320 | $320 |
| Block 17 | $600 | $600 | $340 | $340 |
| Block 18 | $600 | $600 | $360 | $360 |
| Block 19 | $600 | $600 | $380 | $380 |
| Block 20 | $600 | $600 | $600 | $400 |
| Block 21 | $600 | $600 | $600 | $420 |
| Block 22 | $600 | $600 | $600 | $440 |
| Block 23 | $600 | $600 | $600 | $460 |
| Block 24 | $600 | $600 | $600 | $480 |
| Block 25 | $600 | $600 | $600 | $500 |
| Block 26 | $600 | $600 | $600 | $520 |
| Block 27 | $600 | $600 | $600 | $540 |
| Block 28 | $600 | $600 | $600 | $560 |
| Block 29 | $600 | $600 | $600 | $580 |
| Block 30 | $600 | $600 | $600 | $600 |
| 1st of month total ($) | $18,000 | $13,500 | $10,400 | $9,300 |
As a new block drops off each day, it has its usage adjusted to how many days it existed, versus the full month amount. As a result, by month’s end the charges observed on the 1st will have dropped from the initial $18,000 down to the expected $9,300. This number, combined with the newly created backups through the month (which will have a billing table similar to the above, but reversed), will result in a monthly expense in line with last month’s $18,600.
