Sync question

To: lvs-users@xxxxxxxxxxxxxxxxxxxxxx
Subject: Sync question
From: Rodger Erickson <rerickson@xxxxxxxxxxxx>
Date: Tue, 27 Nov 2001 02:07:38 -0800
        I have a few questions about synchronization (ipvs-0.9.7).  Let me
start by saying that even though I've spent a bit of time working with the
LVS source code, I am fully aware that there are certainly subtleties in the
workings of LVS and synchronization that are eluding me.  Thus, please
consider my questions/comments an attempt to learn rather than any soft of
criticism about LVS.

In ip_vs_core.c there is the following fragment of code that regulates the
frequency of synchronization:

/* increase its packet counter and check if it is needed
   to be synchronized */
if (ip_vs_sync_state == IP_VS_STATE_MASTER
    && (cp->protocol != IPPROTO_TCP
        || cp->state == IP_VS_S_ESTABLISHED)
    && (atomic_read(&cp->in_pkts) % 50
        == sysctl_ip_vs_sync_threshold))

        My questions are the following:

        1) Why is "in_pkts % 50 == threshold" used?  This would trigger
synchronization every 50 packets starting with packet number
sysctl_ip_vs_sync_threshold (assuming threshold is less than 50).  Thus it
would seem to me that this threshold value doesn't buy us anything (other
than ignoring synchronization for TCP connections that are very short
lived).  Instead, I would have thought that something like: (
sysctl_ip_vs_sync_threshold && ((atomic_read(&cp->in_pkts) %
sysctl_ip_vs_sync_threshold) == 0)) would be better suited as a conditional
to determine when synchronization of the connection should occur.  Am I
missing something?

        2) Why do connections need to be synchronized on an ongoing basis?
Unless (cp->flags & IP_VS_CONN_F_SEQ_MASK) is true none of the information
being synchronized would ever change (or am I missing something subtle

        3) Thus, it would seem to be more efficient to just synchronize
when a connection is created (for UDP) or enters the established state (for
TCP) or is destroyed.  Why was this approach not used?

        4) As for when (cp->flags & IP_VS_CONN_F_SEQ_MASK) is true, it would
seem to me that we would need to synchronize the connections each time a
packet is received in order for this to work.  Otherwise, it would appear to
be pretty random whether or not the backup would have sufficient information
to handle a failover situation on that particular connection.

        5) There doesn't seem to be any synchronization of persistence
templates.  Is this forthcoming in a future release?

        Also, is there a "roadmap" for synchronization that could help me to
understand the future development plans in this area?

        Thanks very much for helping me to better understand this.

        Rodger Erickson

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