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Re: [Qemu-devel] Re: [PATCH] virtio-spec: document block CMD and FLUSH

From: Jamie Lokier
Subject: Re: [Qemu-devel] Re: [PATCH] virtio-spec: document block CMD and FLUSH
Date: Tue, 4 May 2010 21:32:55 +0100
User-agent: Mutt/1.5.13 (2006-08-11)

Rusty Russell wrote:
> On Fri, 19 Feb 2010 08:52:20 am Michael S. Tsirkin wrote:
> > I took a stub at documenting CMD and FLUSH request types in virtio
> > block.  Christoph, could you look over this please?
> > 
> > I note that the interface seems full of warts to me,
> > this might be a first step to cleaning them.
> ISTR Christoph had withdrawn some patches in this area, and was waiting
> for him to resubmit?
> I've given up on figuring out the block device.  What seem to me to be sane
> semantics along the lines of memory barriers are foreign to disk people: they
> want (and depend on) flushing everywhere.
> For example, tdb transactions do not require a flush, they only require what
> I would call a barrier: that prior data be written out before any future data.
> Surely that would be more efficient in general than a flush!  In fact, TDB
> wants only writes to *that file* (and metadata) written out first; it has no
> ordering issues with other I/O on the same device.

I've just posted elsewhere on this thread, that an I/O level flush can
be more efficient than an I/O level barrier (implemented using a
cache-flush really), because the barrier has stricter ordering
requirements at the I/O scheduling level.

By the time you work up to tdb, another way to think of it is
distinguishing "eager fsync" from "fsync but I'm not in a hurry -
delay as long as is convenient".  The latter makes much more sense
with AIO.

> A generic I/O interface would allow you to specify "this request
> depends on these outstanding requests" and leave it at that.  It
> might have some sync flush command for dumb applications and OSes.

For filesystems, it would probably be easy to label in-place
overwrites and fdatasync data flushes when there's no file extension
with an opqaue per-file identifier for certain operations.  Typically
over-writing in place and fdatasync would match up and wouldn't need
ordering against anything else.  Other operations would tend to get
labelled as ordered against everything including these.

-- Jamie

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