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List:       intermezzo-devel
Subject:    Re: (reiserfs) Re: RFC: Re: journal ports for 2.3? (resendingbecause my ISP probably lost it)
From:       "Peter J. Braam" <braam () cs ! cmu ! edu>
Date:       2000-01-05 15:22:03
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I think I mean joining.  What I need is:

braam starts trans
does A
calls reiser: hans starts
does B
hans commits; nothing goes to disk yet
braam does C
braam commits/aborts ABC now go or don't


- Peter -

On Wed, 5 Jan 2000, Hans Reiser wrote:

> Is nesting really the term you mean to use here, or is joining the term you
> mean?
>
> Do you really mean transactions within other transactions?
>
> Exactly what functionality do you need?
>
> Hans
>
> "Peter J. Braam" wrote:
>
> > Hi,
> >
> > I have one request for the journal API for use by network file systems -
> > it is a request of a slightly different nature than the ones discussed so
> > far.
> >
> > InterMezzo (www.inter-mezzo.org) exploits an existing disk file system as
> > a cache and wraps around it. (Any disk file system can be used, but so far
> > only Ext2 has been exploited.)  High availability file systems need update
> > logs of changes that were made to the cache so that these may be
> > propagated to peers when they come back online (to support "disconnected
> > operation").
> >
> > Requested feature:
> > ------------------------------------------------------------------------
> >
> > Stephen's journal API has a tremendously useful feature: it allows nesting
> > of transactions.   I don't know if Reiser has this (can you tell me
> > Chris?) but it is _incredibly_ useful.  So:
> >
> > - InterMezzo can start a journal transaction
> >  - execute the underlying Ext3 routine within that transaction
> >    (i.e. the Ext3 transaction becomes part of the one started
> >     by InterMezzo)
> > - InterMezzo finishes its routine (e.g. by noting that an update
> > took place in its update log) and commits or aborts the transaction
> >
> > -------------------------------------------------------------------------
> >
> > [So, in particular InterMezzo and Ext3 share the journal transaction log.]
> >
> > Why is this useful? There are at least two reasons:
> >
> >  - the update InterMezzo update log can be kept in sync with the Ext3 file
> > system as a cache
> >
> >  - InterMezzo will soon manage somewhat more metadata (e.g. it may want to
> > remmeber a global file identifier, similar to a Coda FID or NFS file
> > handle) and it can make updates to its metadata atomically with updates
> > made to Ext3 metadata.
> >
> > Both of these reasons touch the core architectural decisions of systems
> > like Coda/AFS/InterMezzo/DCE-DFS -- so there is some historical reason to
> > be so delighted with what one can do with Stephen's API.
> >
> > Presently, systems like Coda and AFS have a hell of a time keeping caches
> > in sync with the metadata and to a large extent Coda's really bad
> > performance is caused by this (an external transaction system is used in
> > conjunction with synchronous operations on the disk file system, ouch...).
> > InterMezzo will start using the kernel journal facility that should be
> > much lighter weight.
> >
> > Is this a reasonable thing to ask for?
> >
> > - Peter -
>
> --
> Get Linux (http://www.kernel.org) plus ReiserFS
>  (http://devlinux.org/namesys).  If you sell an OS or
> internet appliance, buy a port of ReiserFS!  If you
> need customizations and industrial grade support, we sell them.
>

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