This article will introduce DDFS space basic knowledge and VTL tape recommendation.

1.     Understand DDFS space basic knowledge:

When a customer is using the Data Domain in their environment, they will often encounter Data Domain space usage issue.

In the Data Domain command line, use the following command to view Data Domain space usage information:

#filesys show space

/data: Pre-comp line: The original data volume before deduplication.

/data: Post-comp line:

·         Size GiB - All available space

·         Used GiB – The space has been used

·         Avail GiB - Remaining available space

·         Cleanable GiB - Recyclable space

/ddvar line: The private space for Data Domain system (Microcode, logs and other non-user data)

Best practice recommendations:

A: Space usage is less than 80% (When space usage exceeds 80% and 90%, an alarm will occur)

B: Clean up the space once a week (Default setting)

When Data Domain space usage reaches 100%, Data Domain will become a read-only state. So, user backup job will be aborted and it cannot run clean up on Data Domain. Therefore during the daily use of Data Domain, we should be concerned about the space usage to avoid the issue from happening.

2.     VTL tape recommendation:

When using the Data Domain in the real environment, we often encountered that virtual tapes have not been filled, but the Data Domain space was full. The reason for this is mostly due to unreasonable space planning.

For example, Data Domain's available capacity is 10TB. If it is all used for virtual tape library backup, according to the best practice with planning 80% space usage, we can use 8TB. If the customer is using LTO-3 400GB tape, according to the estimates of 10X deduplication rate, it can be configured with 200 tapes:

10TB × 80% × 10 ÷400GB = 200 tapes

However, if after a period of time, we notice it is only 5X deduplication rate. So, if the average space usage rate is 62.5%, or 125 tapes is full, Data Domain systems will reach 100%:

10TB ×5 ÷400GB = 125 tapes

Or

（10TB ×5） ÷（400GB × 200）= 62.5%

It is easy to cause unexpected backup being aborted.

So, when creating tapes, we recommend that according to estimated deduplication rate to create some number of tapes without maximum tapes. After a period of time running backup, then based on actual Data Domain deduplication rate, adjust the planned number of tapes. In addition, we need to plan according to the recommended 80% of the available space instead of 100%.