Network Router User Manual

2 Sun StorEdge Network Data Replicator 3.0 Configuration Guide • June 2001
The replication mode can be synchronous or asynchronous based as a user-selectable
parameter for each Sun SNDR volume set. (Use the sndradm enable command and
select the sync or async parameter Use the sndradm -R m command to change
the replication mode thereafter.) The volumes can be updated synchronously in real
time or asynchronously using a store-and-forward technique.
Typically, a primary volume is first explicitly copied to a designated secondary
volume to establish matching contents. As applications write to the primary volume,
the Sun SDNR software replicates changes to the secondary volume, keeping the two
volumes consistent.
■ In synchronous mode, a write operation is not confirmed as complete until the
remote volume has been updated. Synchronous mirroring forces the Sun SNDR
software to wait until an acknowledgement of the receipt of the data is received
from the secondary by the primary before returning to the application.
■ In asynchronous mode, a write operation is confirmed as complete before the
remote volume has been updated. Asynchronous mirroring allows the Sun SNDR
software to return to the host as soon as the write has been completed on the
primary volume and been placed on a per volume queue for the secondary site.
Subsequently, the secondary site receives the queued requests in the order that
they were queued. Once the I/O has been completed at the secondary site,
notification is sent to the primary.
In the event of planned or unplanned outages, the Sun SNDR software maintains
per-device scoreboard bitmap volumes that are marked to indicate changed blocks
with a granularity of 32 Kbytes per segment. This technique allows for optimized
resynchronization by allowing the Sun SNDR software to resynchronize only the
blocks that have changed.
OtherfeaturesincludedaspartoftheSunSNDRsoftwareare:
■ Grouping volume sets —TheSunSNDRsoftwareenablesyoutogroupvolume
sets. Grouping volume sets guarantees write ordering. For example, in a
synchronization from the primary host to the secondary host, write operations to
the secondary volumes in the group occur in the same order as the write
operations to the primary volumes in the group.
You can also assign specific volume sets to a group to perform sndradm
commands on these volume sets and not on others you have configured. See
“Order-Dependent Writes and Volume Set Grouping” on page 13.
Note – You can also group volume sets according to their cluster or resource tag to
perform replication in a Sun Cluster 2.2 or 3.0 Update 1 clustered environment only.