The syncer rate
configuration parameter
should be configured with care as the synchronization rate can
have a significant effect on the performance of the DRBD setup
in the event of a node or disk failure where the information is
being synchronized from the Primary to the Secondary node.
In DRBD, there are two distinct ways of data being transferred between peer nodes:
Replication refers to the transfer of modified blocks being transferred from the primary to the secondary node. This happens automatically when the block is modified on the primary node, and the replication process uses whatever bandwidth is available over the replication link. The replication process cannot be throttled, because you want to transfer of the block information to happen as quickly as possible during normal operation.
Synchronization refers to the process
of bringing peers back in sync after some sort of outage,
due to manual intervention, node failure, disk swap, or the
initial setup. Synchronization is limited to the
syncer rate
configured for the DRBD
device.
Both replication and synchronization can take place at the same time. For example, the block devices can be being synchronized while they are actively being used by the primary node. Any I/O that updates on the primary node will automatically trigger replication of the modified block. In the event of a failure within an HA environment, it is highly likely that synchronization and replication will take place at the same time.
Unfortunately, if the synchronization rate is set too high, then the synchronization process will use up all the available network bandwidth between the primary and secondary nodes. In turn, the bandwidth available for replication of changed blocks is zero, which means replication will stall and I/O will block, and ultimately the application will fail or degrade.
To avoid enabling the syncer rate
to consume
the available network bandwidth and prevent the replication of
changed blocks you should set the syncer rate
to less than the maximum network bandwidth.
You should avoid setting the sync rate to more than 30% of the
maximum bandwidth available to your device and network
bandwidth. For example, if your network bandwidth is based on
Gigabit ethernet, you should achieve 110MB/s. Assuming your disk
interface is capable of handling data at 110MB/s or more, then
the sync rate should be configered as 33M
(33MB/s). If your disk system works at a rate lower than your
network interface, use 30% of your disk interface speed.
Depending on the application, you may wish to limit the synchronization rate. For example, on a busy server you may wish to configure a significantly slower synchronization rate to ensure the replication rate is not affected.
The al-extents
parameter controls the number
of 4MB blocks of the underlying disk that can be written to at
the same time. Increasing this parameter lowers the frequency of
the meta data transactions required to log the changes to the
DRBD device, which in turn lowers the number of interruptions in
your I/O stream when synchronizing changes. This can lower the
latency of changes to the DRBD device. However, if a crash
occurs on your primary, then all of the blocks in the activity
log (that is, the number of al-extents
blocks) will need to be completely resynchronized before
replication can continue.
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