Dell PowerVault NX3500 - Manual (Page 17)

High Availability And Load Balancing
To optimize availability and performance, client connections are load balanced across the available NAS
controllers. Both NAS controllers in a NAS appliance operate simultaneously. If one NAS controller fails,
clients are automatically failed over to the remaining controllers. When failover occurs, some CIFS clients
reconnect automatically, while in other cases, a CIFS application might fail, and the user must restart it.
NFS clients experience a temporary pause during failover, but client network traffic resumes
automatically.
Failure Scenarios
The FluidFS system can tolerate a NAS controller failure without impact to data availability and without
data loss. If one NAS controller becomes unavailable (for example, because the NAS controller failed, is
turned off, or is disconnected from the network), the NAS appliance status is degraded. Although the
FluidFS system is still operational and data is available to clients, the administrator cannot perform most
configuration modifications and performance might decrease because data is no longer cached.
The impact to data availability and data integrity following a multiple NAS controller failure depends on
the circumstances of the failure scenario. Dell recommends detaching a failed NAS controller as soon as
possible, so that it can be safely taken offline for service. Data access remains intact as long as one of the
NAS controllers in each NAS appliance in a FluidFS system is functional.
The following table summarizes the impact to data availability and data integrity of various failure
scenarios.
Scenario System Status Data Integrity Comments
Single NAS controller
failure
Available, degraded Unaffected
• Peer NAS controller enters
journaling mode
• Failed NAS controller can be
replaced while keeping the
file system online
Sequential dual ‐NAS
controller failure in
single NAS appliance
system
Unavailable Unaffected Sequential failure assumes that
there is enough time between
NAS controller failures to write all
data from the cache to disk (MD
system or non ‐volatile internal
storage)
Simultaneous dual ‐ NAS
controller failure in
single NAS appliance
system
Unavailable Lose data in cache Data that has not been written to
disk is lost
Sequential dual ‐NAS
controller failure in
multiple NAS appliance
system, same NAS
appliance
Unavailable Unaffected Sequential failure assumes that
there is enough time between
NAS controller failures to write all
data from the cache to disk (MD
system or non ‐volatile internal
storage)
Simultaneous dual ‐NAS
controller failure in
multiple NAS appliance
Unavailable Lose data in cache Data that has not been written to
disk is lost
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