External journal on flash drive

Ognyan Kulev ogi at fmi.uni-sofia.bg
Tue Sep 14 04:23:15 UTC 2004

Mika Liljeberg wrote:
> On Tue, 2004-09-14 at 01:02, Stephen C. Tweedie wrote:
>>> or are there blocks that are particularly "hot"?
>>There is one: the journal superblock.  It's not updated _hugely_ often,
>>but it is updated whenever we "checkpoint" the journal (ie. when we
>>remove old transactions from the tail end of the journal.)  I haven't
>>measured it but I'd expect we're updating that maybe 2 or 3 times more
>>rapidly than other journal blocks.
> That's less than ideal but not too bad. Presumably a large journal will
> help here?

Larger journal will be even worse.

I think it's possible the superblock to be updated less: when checkpoint 
transaction is finished, we don't move tail, but remember (in kernel) 
where tail is moved and what superblock remembers.  So we have two 
tails: one in superblock and one in kernel.  The one in superblock is 
moved to the one in kernel only when head of the journal reaches the 
tail that is in superblock.  It should be correct: replaying will just 
replay more transactions.  And it will have performance impact: 
requesting journal block will sometimes require superblock to be 
updated, thus slowing down the transaction a bit.


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