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<H2><A NAME="s4">4. Volumes</A></H2>

<P>Volumes are recorded using an umbrella data structure called the volume database (VDB), defined in venusvol.h.
<P>
<UL>
<LI> volume database.  (persistent)</LI>
<LI>    contains volents (linked), </LI>
<LI> contains mle</LI>
<LI>    resource limits on MLEs</LI>
</UL>
<P>Contained in the volume database are above all entries for individual volumes: volents. Volents are stored in persistent storage. Volents are described in venusvol.h as well and contain the following important fields:
<UL>
<LI>    volume ID (of course)</LI>
<LI>    state (hoarding, emulating, logging (write disconnected),etc.) </LI>
<LI>    flags (state transition pending, reintegrating, repairing, weakly connected, etc.)
counts (readers, writers, etc. for synchronization)</LI>
<LI>    range pre-allocated fids        </LI>
<LI>    version vector, callback status</LI>
<LI>    list of read/write volume ids (if replicated)</LI>
<LI>            replicated volume id (if read-write)</LI>
<LI>    CML</LI>
<LI>    link to VSG entry (vsgent)</LI>
<LI>    list of FSOs</LI>
<LI>    list of COP2 pending (cop2ent)</LI>
<LI>    list of entries requiring resolution (resent)</LI>
<LI>    statistics (vmon, etc.)</LI>
</UL>

XXX What are observers? What are pre-allocated fids?
<P>A volume carries a client modification log.  The umbrella data
structure is the ClientModifyLog, which contains entries of type
cmlent. These are all persistent. A structure of type ClientModifyLog
has an owner, who is responsible for writing all the entries of the
log.  XXX What is an mle versus a cmle
<P>
<UL>
<LI>    list of cmlents</LI>
<LI>    ownership (only one user may own)</LI>
<LI>    statistics</LI>
</UL>
<P>An entry in the CML is a higly technical structure reflecting the filesystem updates. It contains opcodes and a union (u) of data containing the fid and update details. All cmlents are held in RVM.
<P>
<UL>
<LI>    opcode, operands</LI>
<LI>    flags (frozen, local repair, etc.)</LI>
<LI>    links to 
<UL>
<LI>    CML</LI>
<LI>    fsobjs</LI>
<LI>    predecessor, successor</LI>
</UL>
</LI>
</UL>
<P>A very small structure holds the data associated to a cop2 event. This data is transient.
cop2ent -- entry for pending COP2 events        (transient)
<UL>
<LI>    link to volume</LI>
<LI>    store ID</LI>
<LI>    update set</LI>
</UL>
<P>Another small structure holds data associated with fids needing resolution. 
<P>
<UL>
<LI>resent -- entry for fids needing resolution (transient)</LI>
<LI>    fid, result</LI>
</UL>
<P>
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