Clustered Data ONTAP Write Operations

Today one of my technical team asking about ONTAP writing operation in the situation of storage controller going down during normal read/write operation.

If the application send data to controller A with the successful written data to NVRAM, and controller going down and swap (fail-over) to controller B, what’s will happen next in the controller B? Will data continue writing data from controller B to the disk?

So below ONTAP write operation diagram (s) will help to answer this question.
Because NVRAM is running on mirror to the partner node (controller), so once the data already wrote to NVRAM that’s mean both controller’s NVRAM got this update already. Once the fail-over happen, controller B will know this updated in NVRAM. And controller B will be the owner, whose will take responsibility to update written block to the disk in the next interval of disk I/O update.

WRITE Operation (Direct Data Access):

Write operations for direct access take the following path through the storage system:

1. The write request is sent to the storage system from the host via a NIC or an HBA.
2. The write is simultaneously processed into system memory and logged in NVRAM and in the NVRAM mirror the partner node of the HA pair.
3. The write is acknowledged to the host.
4. The write is sent to storage in a consistency point (CP).

WRITE OPERATIONS (INDIRECT DATA ACCESS)

Write operations for indirect data access take the following path through the storage system:

1. The write request is sent to the storage system from the host  via a NIC or an HBA.
2. The write is processed and redirected (via the cluster-interconnect) to the storage controller that owns the volume.
3. The write is simultaneously processed into system memory and logged in NVRAM and in the NVRAM mirror of the partner node of the HA pair.
4. The write is acknowledged to the host.
5. The write is sent to storage in a CP.

WRITE OPERATIONS (FLASH POOL SSD CACHE):

Write operations that involve the SSD cache take the following path through the storage system:

1. The write request is sent  to the storage system from the host via a NIC or an HBA.
2. The write is simultaneously processed into system memory and logged in NVRAM and in the NVRAM mirror of the partner node of the HA pair.
3. The write is acknowledged to the host.
4. The system determines whether the random write is a random overwrite.
5. A random overwrite is sent to the SSD cache; a random write is sent to the HDD.
6. Writes that are sent to the SSD cache are eventually evicted from the cache to the disks, as determined by the
   eviction process).

Reference URL: https://www.unixarena.com/2015/11/netapp-clustered-data-ontap-write-operations-part-5.html/
Credit: UnixArena

-Siam
12 March 2019