Re: [PATCH 0/8][RFC] IO latency/throughput fixes

[Jeff Garzik]

Mark Lord wrote:
> Linus Torvalds wrote:
> > On Tue, 7 Apr 2009, Mark Lord wrote:
> > > What happens with ext3 "writeback", and ext4 "whatever",
> > > when one does the quickie reboot method:
> > >
> > >   ALT-SYSRQ-S ALT-SYSRQ-U ALT-SYSRQ-S ALT-SYSRQ-B
> > >
> > > ???
> >
> > Since 's' syncs (I think 'u' does too, as part of making things 
> > read-only), the data blocks will be on disk after the boot regardless 
> > of any other ordering.
> ..
>
> I was thinking more about delayed allocation in ext4, though.
> If it hasn't allocated the blocks, then sync() has nothing to write out.
> Or do they have hooks into the block layer to force alloc/commit when
> somebody does a sync() ??

sync(2) doesn't just sync dirty buffers...  it sync's inodes, which 
pokes the filesystem to do something intelligent, perhaps triggering (a) 
write-out of data, (b) write-out of zeroed blocks, or (c) annotation in 
filesystem metadata that certain blocks are allocated, but not initialized.

	Jeff



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