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Re: sync writes - was Re: how to counteract slowdown

Neil Brown wrote:
> On Tuesday November 13, akpm zip com au wrote:
> > >
> > > After the fsync call, where is the content of buffer? Is it:
> > > (a) in the journal only
> > > (b) in the journal and on disk
> > >
> > > This is the one bit that confuses me from your answer.
> >
> > After the fsync that data is in the journal, and it is
> > in memory.  Once the in-memory copy has been written out
> > to the main filesystem, its journal space can be recycled.
> >
> I thought I would double check this.  So I watched the number of SCSI
> operations on my scsi drives while doing various sorts of sync writes
> to variously mounted filesystems.
> ...

I think your numbers fail to account for request
merging?  A write of 40000 bytes to a data=journal
fs should result in two requests at the SCSI layer,
and that's what you (sometimes) get.

> ...
> ext3 sync,data=journal
>      async         0  200       sync mount option now honoured!
>      fsync         0  300

These need thinking about.

>      osync         0    2       but O_SYNC negates effect of -o sync!!

That's expected - one request is the data+metadata into the
journal.  The other is the commit block.

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