User's Guide

Fence Level of ASYNC
Fence level = ASYNC is recommended to improve performance in data replication between the
primary and the remote site.
The XP disk array supports asynchronous mode with guaranteed ordering. When the host does a
write I/O to the XP disk array, as soon as the data is written to cache, the array sends a reply to
the host. A copy of the data with a sequence number is saved in an internal buffer, known as the
side file, for later transmission to the remote XP disk array. When synchronous replication is used,
the primary system cannot complete a transaction until a message is received acknowledging that
data has been written to the remote site. With asynchronous replication, the transaction is completed
once the data is written to the side file on the primary system, which allows I/O activity to continue
even if the Continuous Access link is temporarily unavailable.
The side file is 30% to 70% of cache (default 50%) that is assigned through the XP system’s Service
Processor (SVP). The high water mark (HWM) is 30% of the cache as shown in Figure 28. However,
if the quantity of data in the side file exceeds 30% then the write I/O to the side file will be delayed.
The delay can be from .5 seconds to a maximum of 4 seconds, in 500 ms increments, with every
5% increase over the HWM.
If the HWM continues to grow, it will eventually hit the side file threshold of 30% to 70% cache.
When this limit has been reached, the XP on the primary site cannot write to the XP on the secondary
site until there is enough room in the side file. The primary XP will wait until there is enough room
in the side file before continuing to write. Furthermore, the primary XP will keep trying until it
reaches its side file timeout value, which is configured through the SVP. If the side file timeout has
been reached, then the primary XP disk array will begin tracking data on its bitmap that will be
copied over to the secondary volume during resync.
Figure 28 depicts the side file operation.
Figure 28 XP Series Disk Array Side File
NOTE: The side file must be configured using the XP Service Processor (SVP). Refer to the XP
Series documentation for details. The P9000 disk array family does not support asynchronous
replication using side file.
In case all the Continuous Access links fail, the remaining data in the side file that has not been
copied over to the SVOL will be tracked in the bit map. The application continues to modify the
data on the PVOL, which will also be tracked in the bit map. The SVOL only contains a copy of
the data up to the point the failure of the Continuous Access links. If an additional failure, such as
a system failure before the Continuous Access link is fixed, causes the application to fail over to
the SVOL side, the application will have to deal with non-current data.
Continuous Access Link Timeout
In asynchronous mode, when there is an Continuous Access link failure, both the PVOL and SVOL
sides change to a PSUE state. When the SVOL side detects missing data blocks from the PVOL
side, it will wait for those data blocks from the PVOL side until it has reached the configured
Continuous Access link timeout value (set in the SVP). Once this timeout value has been reached,
then the SVOL side will change to a PSUE state. The default Continuous Access link timeout value
is 5 minutes (300 seconds).
156 Building Disaster Recovery Serviceguard Solutions Using Metrocluster with Continuous Access for P9000 and XP