Memory consideration using buffer and buffer.toString() in node.js streams

Question

I am performing readstream.pipe(transformstream).pipe(writeStream). The application is read xml from a file system using readstream --> do some manipulation based on different tags using transform stream --> write it to another file system.

During the transform operation inside the transformstream. I am doing buffer.toString() for every chunk as I need to manipulate string before pushing them:

      myTransformStream._transform = function(chunk, encoding, done) {
        var stringTobeManipulated = chunk.toString();
        // perform changes on stringTobeManipulated and push it to writestream
      }

the size of xmls can be upto 3 mb and I noticed I got divided to ~44 chunks.

Based on the given problem I have questions concerning memory consumption both in v8 heap and system:

1) I understand that buffers are stored outside the v8 memory. When I do a chunk.toString() in my _transform function, does it create a javascript string object inside the v8 memory ? If yes, I am assuming it will be garbage collected after it looses all its references.

2) As the buffers are part of the system memory I believe they are not garbage collected, so when is that memory freed up ?

3) Is the application a good use case of transform stream as I am converting every chunk to string ?

EDITED: May be I am not explaining myself clearly. Anyways I have been trying to find a ay of removing namespace from the xml tag before converting it to json. Here is the code I ended up with. Took advantage of memoization in javascrtipt. Please let me know if you guys find a better or more memory efficient way. We are using actionhero.js framework

var action = {};
var xml2js = require('xml2js');
var fs = require('fs');
/////////////////////////////////////////////////////////////////////
// metadata
action.name = 'removeNamespace';
action.description = 'I will parse the article related xml and yield the result';
action.inputs = {
    'required': [],
    'optional': []
};
action.blockedConnectionTypes = [];
action.outputExample = {
    articleHeading: 'heading',
    articleBody: 'body'
};

/////////////////////////////////////////////////////////////////////
// functional
action.run = function(api, connection, next) {
    var stream = require('stream');
    var removeNamespace = new stream.Transform();
    var util = require('util');
    var output = fs.createWriteStream('output.xml');
    removeNamespace._transform = function(chunk, encoding, done) {
        removeNamespace._transform.brokenTag = removeNamespace._transform.brokenTag || false;
        removeNamespace._transform.brokenString = removeNamespace._transform.brokenString || '';
        var convertedString = chunk.toString().trim();
        if (removeNamespace._transform.brokenTag === true){
                convertedString = removeNamespace._transform.brokenString + convertedString ;
        }
        removeNamespace._transform.brokenTag = false;
        removeNamespace._transform.brokenString = '' ;
        if (convertedString.lastIndexOf('<') > convertedString.lastIndexOf('>') ){
            removeNamespace._transform.brokenString =convertedString.substring(convertedString.lastIndexOf('<'),convertedString.length+1);
            convertedString = convertedString.substring(0,convertedString.lastIndexOf('<')) ;
            removeNamespace._transform.brokenTag = true ;
        }
        convertedString = convertedString.replace(/>\s+</g, '><')        
                                         .replace.replace(/<[A-Za-z]+:/gi, '<')
                                         .replace(/<\/[A-Za-z]+:/gi, '</');
        done(null, convertedString);
    };
    var Writable = stream.Writable;

    function xmlInMemory(keyToXml, options) {
        Writable.call(this, options);
        this[keyToXml] = new Buffer('');
        this._write = function(chunk, encoding, callback) {
            chunk = (Buffer.isBuffer(chunk)) ? chunk : new Buffer(chunk, encoding);
            this[keyToXml] = Buffer.concat([this[keyToXml], chunk]);
            callback();
        };
    }
    util.inherits(xmlInMemory, Writable);
    var source = fs.createReadStream('path/to/your/xmlfile');
    var target = new xmlInMemory('keyToXml');
    source.pipe(removeNamespace).pipe(target);
    target.on('finish', function() {
        var parser = new xml2js.Parser();
        connection.response.xml2js = target.keyToXml.toString();
        next(connection, true);
        parser.parseString(target.keyToXml.toString(), function(err, result) {  
            connection.response.xml2js = result;
            next(connection, true);
         });
    });
};

/////////////////////////////////////////////////////////////////////
// exports
exports.action = action;