[Tux3] forward logging and NVRAM

[Daniel Phillips]

Hi Timothy,

On Sunday 27 July 2008 14:10, timothy norman huber wrote:
> Daniel,
> 
> Several weeks ago, during a discussion over coffee, you mentioned a  
> rather intriguing optimization- by writing the start of the forward  
> log chain to NVRAM rather than disk, you avoid a disk seek and write  
> transfer for each forward log chain.  i believe there was another  
> motivation that had to do with avoiding traversing the entire volume  
> in case of a crash.  Was this design feature to overcome lame fsck  
> performance?

The same motivation, actually.  Writing the start of a forward log
chain to nvram instead of to some known location on disk means that I
do not have to worry about doing unnatural things to optimize such
loads as O_SYNC writing, where each transaction has to complete
before the next one can begin, which means that the forward log chain
never gets more than one element log, requiring a seek to the known
location for every transaction.  Not worse than journalling to be
sure, and usually better, but not as fast as avoiding a seek and disk
write per transaction entirely.

So if somebody could give me 16 bytes of NVRAM or so per volume I
would be most pleased and could use that to generate some nice O_SYNC
benchmarks :-)

By the way, I wonder why your posts do not show up in the Mailman
(Pipermail) archive, while mine and Matt's do?

Daniel

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