By default, the Axcient Virtual Manager (AVM) attempts to perform an application-consistent snapshot of protected devices. Application-consistent snapshots are typically the most desired form of replication.
Sometimes, when the ESX is under heavy load or if there are issues with the VSS writers, VMware is unable to quiesce the database. When this happens, we are unable to take an application-consistent snapshot and instead take a crash-consistent snapshot.
The type of snapshot taken does not affect the amount of time required for the replication to complete.
The snapshot type only affects (a) how the data is replicated and (b) the system state of the protected device when it is recovered.
An application-consistent snapshot uses VSS writers, which the application(s) respond to by flushing all memory content and I/O operations, ensuring that the application is consistent. Buffers are flushed, operations are completed, files are closed, and so on, resulting in no data being left in memory.
- This type of snapshot is most suited for database applications.
- No operations are lost in the application-consistent snapshot process.
- Once the snapshot is complete, the VSS writers tell the application to resume normal operations.
- An application-consistent snapshot takes the most up-to-date snapshot of the device.
- An application-consistent snapshot includes (rather than ignores) any in-process operations in the replication.
A crash-consistent snapshot means although data is preserved, any transaction in memory (and not committed to the disk) will be lost.
- While this type of replication generally works for many cases, it is not preferred when database applications are involved.
- A crash-consistent snapshot replicates the device data as-is.
- When recovered, a crash-consistent shapshot will restore data in the same system state as when the snapshot was taken.
- No pending operations, open files or unfinished business are replicated or restored in a crash-consistent shapshot.
- If there is data in the memory, or if there are I/O operations in process or pending, the crash-consistent snapshot will ignore that data and those operations.
- Simply put, a crash-consistent shapshot only replicates actual data on the device, and does not replicate any pending operations. The restore consists solely of the system state, as if the device was unplugged and then plugged back in.