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Re: [Xen-users] xenstore-write size limit

> I have been playing with ClickOS https://github.com/cnplab/clickos
> It is a Click modular router running on (MiniOS based) Xen VM (domU).
> In order to run Click, one needs to write Click configuration to the domU
> via xenstore-write. While I've got simple configurations up-and-running,
> I'm struggling to run NAT configuration
> https://github.com/cnplab/clickos/blob/master/conf/mazu-nat.click
> I guess the problem is to do with a data size I'm writing via
> xenstore-write. When I write entire mazu-nat.click file (~7 KB) via
> $ xenstore-write /local/domain/$DOMID/clickos/0/config/0 "`cat
> mazu-nat.click`"
> it gives me a following error message (the same appears
> in /var/log/xen/xen-hotplug.log)
> "xenstore-write: could not write path /local/domain/13/clickos/0/config/0"
> However, when I reduce the mazu-nat.click size (by deleting some lines)
> down to around 4 KB, xenstore-write does not give any error message and VM
> runs successfully. But in order to run NAT logic correctly, I need to write
> entire Click configuration (mazu-nat.click it is around ~5 KB even after
> all redundant lines/comments are deleted).
> I could not find any relevant information from xenstore reference at
> http://wiki.xen.org/wiki/XenStore_Reference nor from the mailing list about
> maximum size of data possible to write via xenstore-write. Can someone
> please help?


The payload length (len field of the header) is limited to 4096
(XENSTORE_PAYLOAD_MAX) in both directions.  If a client exceeds the
limit, its xenstored connection will be immediately killed by
xenstored, which is usually catastrophic from the client's point of
view.  Clients (particularly domains, which cannot just reconnect)
should avoid this.


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