ForceImport
Hi all,
I need help to good understand attribute ForceImport.
There is a table "Failure situations" in Symantec Storage Foundatio and High Availability Solutions Solutions Guide. (221p.)
I don't understant situation number 7 Split-brain situation andstorage interconnect lost.
ForceImport set to 0 - No interruption of service.Can’t import with only 50% of disks available. Disks on the same node are functioning. Mirroring is not working.
ForceImport set to 1 - Automatically imports disks on secondary site. Now disks are online in both locations—data can be kept from only one.
Who can unswer this question:
"Now disks are online in both locations—data can be kept from only one" What is the purpes? What we will get if first 50% of disks will imported on 1 site and second 50% will imported on 2 site?
The ForceImport is most applicable in campus clusters when you have one or more nodes at site A in a cluster with one or more nodes at site B using a diskgroup which has mirrored volumes using an array at site A and an array at site B.
So consider the following 4 scenarios:
- Site A goes down
- Array at Site B does down
- SAN Fibre channel is lost between sites
- SAN Fibre channel and network is lost between sites
With ForceImport set to 1 the in the 4 sceanrios you have:
- Able to import DG on site B
- Still able to import DG at site A for local failover or if reboot server at site A
- Still able to import DG at either site - VCS will not let you import both at same time, but you could potentially write some data at site A, then fail to site B where you won't have data written at site A and you can still continue to write data at site B, so you can potentially end up having to manually merge data written at both sites or (more likely) have to discard data from one of the sites
- Still able to import DG at either site and in fact in most VCS configurations, VCS will automatically import on the inactive site so you will then have DG imported on both sites, each with half of the disks, so then, as is 3, you end up having to manually merge data written at both sites or (more likely) have to discard data from one of the sites
With ForceImport set to 0 the in the 4 sceanrios you have:
- Not able to import DG on site B
- Not able to import DG at site A for local failover or if reboot server at site A
- Still able to import DG at either site
- Still able to import DG at either site
So in essence ForceImport=0 protects against your mirrored volumes writing independently to each mirror (split-brain) at the expense that you will have to take manual steps to import the diskgroup when only half the disks are available.
Personnally I always set ForceImport=1 as there are other ways to protect against split-brain such as:
a. Use GCO with ClusterFailOverPolicy = CONNECTED
b. Use Replicated Data Cluster with AutoFailOver set to 2 (manual failover acrosss sites)
c. Use PreOnline script to code your own logic as to when SG is allowed to online
d. Make DIskgroup resource dependent on IP resource in VCS, so then if public network is up between sites (even is LLT is down) the IP resource will not online if it is online on the other site and hence the diskgroup never gets to online.
Mike