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Re: WAPBL vs. lfs?

On Fri, Aug 22, 2008 at 04:54:06PM -0400, Thor Lancelot Simon wrote:
> However, LFS in 4.x can use between 75% and 100% of the disk bandwidth for
> writes whether those are small file writes, directory creates, removes,
> you name it.  Try the same thing with WAPBL and you'll see that though it
> is a lot faster than standard FFS, it's still able to use well under half
> the maximum throughput of the disk in many such cases -- and that is even
> if you don't subtract the extra-writes penalty for the journal (all
> metadata is effectively written twice).

To be fair, the extra penalty you mention for WAPBL is not that large as
the journal entries are consecutive storage on the disk and need only a
single seek per flush. Especially if the journal can keep more than one
version of a disk block (consider cvs up), it is a lot better on the
disk as it doesn't force the ordered writes as softdep does. So saying
that all metadata is written twice isn't excatly true and even if it
does happen to be, wapbl allows the disk scheduler to work more
efficient as it breaks the dependency cycles.


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