aeron-client/src/main/java/io/aeron/Subscription.java
public long blockPoll(final BlockHandler blockHandler, final int blockLengthLimit)
    {
        long bytesConsumed = 0;
        for (final Image image : images)
        {
            bytesConsumed += image.blockPoll(blockHandler, blockLengthLimit);
        }

        return bytesConsumed;
    }
public long blockPoll(final BlockHandler blockHandler, final int blockLengthLimit)
    {
        long bytesConsumed = 0;
        for (final Image image : images)
        {
            bytesConsumed += image.blockPoll(blockHandler, blockLengthLimit);
        }

        return bytesConsumed;
    }
Poll the {@link Image}s under the subscription for available message fragments in blocks.
<p>
This method is useful for operations like bulk archiving and messaging indexing.

@param blockHandler     to receive a block of fragments from each {@link Image}.
@param blockLengthLimit for each {@link Image} polled.
@return the number of bytes consumed.