[Lustre-devel] global epochs

[Peter J. Braam]

Hi -

Nikita's discussion is excellent.  I read through and thought about this a
bit over the vacation.   Two important issues remain unclear in my mind:

1. the write back cache mode is not good for scaling (eg. for many nodes
creating files in one directory); for that intents save a lot of RPCs.  The
recovery correctness discussion doesn't cover this, and I've found this to
introduce new opportunities for cascading aborts, that are not managed by
client locks.  I think it would be most beautiful if the epoch protocol was
somewhat enhanced to also handle intent style processing (where there is
dependence on RPC responses).  I think some of the response data (FIDs
notably) is not protected by locks (as it is immutable).

2. I'm nervous about broadcasting epoch messages to, for example, 1 million
clients (I think that is the current target).  You'd want a tree model for
this, and do careful calculations how often failures will occur and what
their impact is on recovery time.

Peter
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