ehcache config for Shiro's PropertiesRealm-0-accounts

[Charles Syperski]

I am attempting to get shiro working with terracotta for session 
replication.  I think everything is setup and good to go with the 
exception of my cache settings on the client side (ehcache.xml) for 
'org.apache.shiro.realm.text.PropertiesRealm-0-accounts'

Here is my current config for this cache:
<cache name="org.apache.shiro.realm.text.PropertiesRealm-0-accounts"
           maxElementsInMemory="10000"
           eternal="true"
           overflowToDisk="false"
           diskPersistent="false">
        <terracotta>
            <nonstop immediateTimeout="false" timeoutMillis="30000">
                <timeoutBehavior type="exception" />
            </nonstop>
        </terracotta>
    </cache>

My questions are, first, what is this cache used for (sorry for the 
newbie question)?  I couldn't find much documentation on this, if 
someone could just point me to a link that would be awesome. Second, do 
I need to specify any TTL or expiry settings like:

timeToLiveSeconds="0"
timeToIdleSeconds="0"
diskExpiryThreadIntervalSeconds="600"

Thanks for any help in advance!

Thanks,
Chuck Syperski