Specifications

Avoiding Full Resynchronizations

When replicating large amounts of data over a WAN link, it is desirable to avoid full

resynchronizations which can consume large amounts of network bandwidth and time. With newer

kernels, SteelEye DataKeeper can avoid almost all full resyncs by using its bitmap technology.

However, the initial full resync, which occurs when the mirror is first set up, cannot be avoided when

existing data is being replicated. (For brand new data, SteelEye does not perform a full resync, so the

steps below are not necessary.)

There are a couple of ways to avoid an initial full resync when replicating existing data. Two

recommended methods are described below.

Method 1

The first method consists of taking a raw disk image and shipping it to the target site. This results in

minimal downtime as the mirror can be active on the source system while the data is in transit to the

target system.

Procedure

1. Create the mirror (selecting Replicate Existing Filesystem), but do not extend the mirror to the

target system.

2. Take the mirror out of service.

3. Take an image of the source disk or partition. For this example, the chosen disk or partition is

/dev/sda1:

root@source# dd if=/dev/sda1 of=/tmp/sdr_disk.img bs=65536

(The block size argument of 65536 is merely for efficiency).

This will create a file containing the raw disk image of the disk or partition.

Note that instead of a file, a hard drive or other storage device could have been

used.

4. Optional Step – Take a checksum of the source disk or partition:

root@source# md5sum /dev/sda1

5. Optional Step – Compress the disk image file:

root@source# gzip /tmp/sdr_disk.img

6. Clear the bitmap file, e.g.:

root@source# /opt/LifeKeeper/bin/bitmap -c

/opt/LifeKeeper/bitmap_sdr

304Administration