Sun Storage 7000 Unified Storage System: How much free space to leave when configuring a pool with LUNs

Asset ID:	1-79-1995759.1
Update Date:	2018-01-08
Keywords:

Solution Type Predictive Self-Healing Sure

Solution 1995759.1 : Sun Storage 7000 Unified Storage System: How much free space to leave when configuring a pool with LUNs

Applies to:

Oracle ZFS Storage ZS3-2 - Version All Versions and later
Oracle ZFS Storage ZS4-4 - Version All Versions and later
Sun Storage 7210 Unified Storage System - Version All Versions and later
Sun ZFS Storage 7420 - Version All Versions and later
Sun ZFS Storage 7320 - Version All Versions and later
7000 Appliance OS (Fishworks)

Purpose

The purpose of this document is to advise about free space reservation as we configure LUNs. We need to allow enough space for metadata so that the pool on the ZFS Storage Appliance will not fill up before the LUN fills up.

Scope

There is a bug on the appliance which does not allow ZFSSA to calculate the right refreservation value for LUNs.

Details

When configuring a pool containing at least one LUN, customers need to reserve enough free space to allow metadata and data to be both written and accessed as the pool fills up. The total space to reserve is calculated this way :

Total space to reserve = (1+[256/volblocksize]) * volsize

with :
volsize : size of the lun we configure.
volblocksize : blocksize selected for the volume, at creation time.

1- Example with the default blocksize, 8K

Let's take the example of a LUN of 1G (volsize = 1G), made of blocks of 8K (volblocksize = 8K). The total space to reserve is as follows :

(1+ [256/8192]) * 1GB --> 1,0312 GB

This is 3% of the total size of the LUN.

2- Example of a big lun/small block size

For a very big LUN of 1T, with a volblcsize of 512 bytes (extreme case). The total space to reserve is as follows :

(1+256/512) * 1TB --> 1536 GB

This is 50% of the total size of the LUN.

3- Conclusion

For LUNs with a default volblock size of 8KB, it is safe to leave 3% of the size of the luns. In practice, if a 1TB pool is made of 2 LUNs sized equally, using the default block size (8K), we will configure luns of 435G each in size. This will leave 30GB for metadata to be written in the pool. If we choose a smaller block size, then we need to recalculate the percentage value, which can reach 50% for the smallest block size available on the BUI (512 bytes). That recommendation does not apply for shares, we do not need to take any particular care about space reserved.

Associated bugs are 15819633 and 15785991.

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