It
happens alot of time with me in the bay while working what kind of snapshot we
choose to restore machine or recover machine with , well there was the time
when i wasn't aware and i was like doesn't matter because in my cases it always
worked but i was wrong and yes there is difference between them and has
different use cases.
So
will see what the difference between them and what should we use -
Crash-consistent
As
the name suggests this snapshot is pretty good if your machine got crashed or
you want to recover machine either via recovery services during ASR replication
or restore from backup because this is the snapshot of your disk.
It
captures data at that particular time which was on the disk.
It
doesn't include anything in-memory
This
doesn't guarantee data consistency for the OS or for apps on the VM.
*
Site Recovery creates crash-consistent recovery points every 5 mins by default
& cant be modified.
Most of the apps can easily recover well from crash consistent points, its usually sufficient for replicating OS , apps like DHCP servers and print servers.
App-Consistent
As
name suggest its more app oriented snapshots that means it contains all the
data that crash-consistent contains plus all the data in memory and
transactions in progress.
App-consistent
snapshot uses Volume Shadow Copy Service (VSS)
When
a snapshot initiated, VSS performs a copy-on-write ( COW ) operation on the
volume & before it perform the COW , VSS informs every app on the machine
that it needs to flush its memory-resident data to disk.
App-consistent
snapshot is complex as compare to others and takes longer time.
They
affect the performance of apps running on a VM enabled for replication.
*
frequency should always be less than you set for retaining recovery points.
ASR uses the above mentioned recovery points or snapshots :
Recovery
points are created from snapshots of VM disks taken at a specific point in
time. When you fail over a VM, you use a recovery point to restore the VM in
the target location.
When
failing over, we generally want to ensure that the VM starts with no corruption
or data loss, and that the VM data is consistent for the operating system, and
for apps that run on the VM. This depends on the type of snapshots taken.
(ASR)
Site Recovery takes snapshots as follows:
- Site Recovery takes crash-consistent
snapshots of data by default, and app-consistent
snapshots if you specify a frequency for them.
- Recovery points are created from the
snapshots, and stored in accordance with retention settings in the
replication policy.
Check replication process and policy with respect to Azure 2 Azure DR
https://pachehra.blogspot.com/2019/06/asr-rep.html
https://pachehra.blogspot.com/2019/06/asrconnreq.html
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