That reminds me of structures called "BITMAP.SYS" and "VOLUME.SYS"
directory
names like SYS$SYSTEM:[SYSEXE], but unfortunately, those things belong to
the
forgotten past in a galaxy far, far away where Billy Bitsandbites was
fighting
VMS demons.
--
Mladen Gogala
Oracle DBA
> -----Original Message-----
> From: ml-errors_at_fatcity.com [mailto:ml-errors_at_fatcity.com] On
> Behalf Of Matthew Zito
> Sent: Friday, September 19, 2003 1:15 PM
> To: Multiple recipients of list ORACLE-L
> Subject: RE: asynch I/O
>
>
>
> Well, the other semi-unique thing about WAFL is the fact that
> all meta-data stored in files, rather than custom structures.
> So, the "tree" I was describing is actually three files-
> the free inode map, the free space map, and the inode file.
> The most important one for our purposes here is the inode
> file - that describes the actual filesystem structure.
>
> Sooo, what happens is that the inode file is modified every
> time the structure of the filesystem is changed. Practically
> speaking, this means it is cache-resident all the time. But,
> since WAFL's architecture is to never update blocks, the
> inode file is never updated on disk - a new one is simply written.
>
> This is how Netapp snapshots work - basically there is a
> "root inode" that is special and points to the inode file.
> When you want to make a snapshot, you make a new "root inode"
> and statically point it at the current inode file. Since
> that inode file describes the view of the filesystem _at that
> point in time_, you end up with a read-only virtual filesystem.
>
> This same logic is applied to insuring on-disk consistency.
> Every few seconds, a new snapshot is created that points at
> the current inode file. The netapp continues processing
> requests, but the on-disk filesystem structure is "fixed" at
> that snapshot. When the consistency timer expires, the old
> snapshot is deleted and a new one created that points at the
> current inode file - so the entire filesystem view on-disk
> updates atomically to represent what the Filer had already
> been representing in memory.
>
> The battery-backed cache stores all of the transactions
> between the last consistency point and the present moment (in
> another unique note, it actually caches the NFS operation
> itself, not the low-level I/O). This gives it a pool of
> marked-as-completed writes to work with to help make more
> intelligent decisions about write layouts.
>
> So, the situation is not so grim as "lose your cache, lose
> your filesystem"
> - in a truly tragic scenario with power failure plus
> cache-battery failure, the worst case is that you would
> recover your filer to discover that it was at a consistent
> state from 10 seconds before the power failure (10 seconds is
> the longest time a filer will go between consistency points).
>
> Thanks,
> Matt
> *still pleased with netapp's craftiness*
>
> --
> Matthew Zito
> GridApp Systems
> Email: mzito_at_gridapp.com
> Cell: 646-220-3551
> Phone: 212-358-8211 x 359
> http://www.gridapp.com
>
> > -----Original Message-----
> > From: ml-errors_at_fatcity.com [mailto:ml-errors_at_fatcity.com] On
> > Behalf Of Tanel Poder
> > Sent: Friday, September 19, 2003 12:35 PM
> > To: Multiple recipients of list ORACLE-L
> > Subject: Re: asynch I/O
> >
> >
> > > available raid stripe that's free and writes the block there, then
> > > updates the tree. Besides being rather crafty, it creates
> > a situation
> > > where
> >
> > And the tree is living in batter backed cache?
> >
> > Tanel.
> >
> >
> > --
> > Please see the official ORACLE-L FAQ: http://www.orafaq.net
> > --
> > Author: Tanel Poder
> > INET: tanel.poder.003_at_mail.ee
> >
> > Fat City Network Services -- 858-538-5051 http://www.fatcity.com
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> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.net
> --
> Author: Matthew Zito
> INET: mzito_at_gridapp.com
>
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--
Author: Mladen Gogala
INET: mladen_at_wangtrading.com
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San Diego, California -- Mailing list and web hosting services
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Received on Fri Sep 19 2003 - 12:25:01 CDT
