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Dell PowerVault MD3600f/MD3620f Remote Replication Functional Guide
Page 8
NOTE
:
The write completion signal is sent in Step 3 as opposed to Step 7 in synchronous mode
as shown in
Figure 3.
Asynchronous replication is usually required when the remote site distances are greater than 10KM
and cannot tolerate slowing the host application I/Os by waiting for remote write completions.
The number of write I/Os supported by asynchronous replication is distance determined. The usual
rule of thumb is to allow 1ms for each 100KM in distance. Therefore a 1000KM (621 miles) distance will
require 10ms for a round-trip RR I/O and will, therefore, limit the numbers of IOPs to 100. Longer than
1000KM distances will proportionally reduce this IOPs limit. It is imperative that the distance between
MD36X0f storage arrays be a critical design factor that is surfaced early in the design and accounted
for in the implementation of remote replication.
There are several reasons for highlighting the replication latencies. First, peak period writes (in IOPs)
will determine whether RR can keep its replication queue from exceeding the queue limits. If the
communications link cannot handle the peak period IOPs, and if the queues are then exceeded, then
RR will suspend replication and go into suspend mode. We will take a deeper look into this later.
Asynchronous Replication with Write Order Consistency
For some applications such as database applications, out of order write I/Os will cause database
recovery to fail and lead to data inconsistency and data loss. Databases operate under a strict
sequencing protocol of writes to tablespaces and to logs. The logs are used to ensure the consistency
or correctness of the database transactions as they occur and especially when they complete or
commit. The order of the writes to each of the database tablespace objects that reside on virtual disks
must also be the same order the remote writes are applied at the remote MD36X0f. This includes the
same relative order within the same virtual disk and between the tablespace virtual disks and the log
files.
Replication write activity in a synchronous mode guarantees the same write completion order on
the remote array, providing the best chance of data recovery from the remote site. With
asynchronous write mode, replication write requests are issued in parallel, and therefore not
guaranteed to be sent and/or completed in the same order as received by the primary array.
In order to ensure that all writes to the secondary virtual disks are in the same exact order as they
were applied to the local primary virtual disks, RR provides an Asynchronous Write Order
Consistency Replication (AWOC) mode. The collection of virtual disks configured for write
consistency is often referred to as a consistency group. AWOC provides a consistency group in
which all associated virtual disks that need remote writes to be completed in the same order as
the primary writes occurred are included. RR then ensures that all the writes for this consistency
group’s virtual disks are sent and written in absolute in-order sequence. Only one write
consistency group is allowed per array.
In Figure 4
, when host issues writes X, Y and Z to the primary site array, preserved write order
consistency ensures write requests are issued to the remote array in the same order as on the
local array. So, the remote array receives the write request in the same X, Y and Z order as the
primary system did.