Re: Issues with delalloc->real extent allocation

From: Geoffrey Wehrman <gwehrman@sgi.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@infradead.org>, bpm@sgi.com, xfs@oss.sgi.com
Subject: Re: Issues with delalloc->real extent allocation
Date: Fri, 21 Jan 2011 08:41:52 -0600	[thread overview]
Message-ID: <20110121144152.GD10729@sgi.com> (raw)
In-Reply-To: <20110121025140.GY16267@dastard>

On Fri, Jan 21, 2011 at 01:51:40PM +1100, Dave Chinner wrote:
| Realistically, for every disadvantage or advantage we can enumerate
| for specific workloads, I think one of us will be able to come up
| with a counter example that shows the opposite of the original
| point. I don't think this sort of argument is particularly
| productive. :/

Sorry, I wasn't trying to be argumentative.  Rather I was just documenting
what I saw as potential issues.  I'm not arguing against your proposed
change.  If you don't find my sharing of observations productive, I'm
happy to keep my thoughts to my self in the future.

| Instead, I look at it from the point of view that a 64k IO is little
| slower than a 4k IO so such a change would not make much difference
| to performance. And given that terabytes of storage capacity is
| _cheap_ these days (and getting cheaper all the time), the extra
| space of using 64k instead of 4k for sparse blocks isn't a big deal.
| 
| When I combine that with my experience from SGI where we always
| recommended using filesystems block size == page size for best IO
| performance on HPC setups, there's a fair argument that using page
| size extents for small sparse writes isn't a problem we really need
| to care about.
| 
| І'd prefer to design for where we expect storage to be in the next
| few years e.g. 10TB spindles. Minimising space usage is not a big
| priority when we consider that in 2-3 years 100TB of storage will
| cost less than $5000 (it's about $15-20k right now).  Even on
| desktops we're going to have more capacity that we know what to do
| with, so trading off storage space for lower memory overhead, lower
| metadata IO overhead and lower potential fragmentation seems like
| the right way to move forward to me.
| 
| Does that seem like a reasonable position to take, or are there
| other factors that you think I should be considering?

Keep in mind that storage of the future may not be on spindles, and
fragmentation may not be an issue.  Even so, with SSD 64K I/O is very
reasonable as most flash memory implements at a minimum 64K page.  I'm
fully in favor your proposal to require page sized I/O.

-- 
Geoffrey Wehrman

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