Re: [Lsf-pc] [LSF/MM TOPIC] really large storage sectors - goingbeyond 4096 bytes

From: James Bottomley
Date: Wed Jan 22 2014 - 12:00:45 EST

On Wed, 2014-01-22 at 11:45 -0500, Ric Wheeler wrote:
> On 01/22/2014 11:03 AM, James Bottomley wrote:
> > On Wed, 2014-01-22 at 15:14 +0000, Chris Mason wrote:
> >> On Wed, 2014-01-22 at 09:34 +0000, Mel Gorman wrote:
> >>> On Tue, Jan 21, 2014 at 10:04:29PM -0500, Ric Wheeler wrote:
> >>>> One topic that has been lurking forever at the edges is the current
> >>>> 4k limitation for file system block sizes. Some devices in
> >>>> production today and others coming soon have larger sectors and it
> >>>> would be interesting to see if it is time to poke at this topic
> >>>> again.
> >>>>
> >>> Large block support was proposed years ago by Christoph Lameter
> >>> ( I think I was just getting started
> >>> in the community at the time so I do not recall any of the details. I do
> >>> believe it motivated an alternative by Nick Piggin called fsblock though
> >>> ( At the very least it would be nice to
> >>> know why neither were never merged for those of us that were not around
> >>> at the time and who may not have the chance to dive through mailing list
> >>> archives between now and March.
> >>>
> >>> FWIW, I would expect that a show-stopper for any proposal is requiring
> >>> high-order allocations to succeed for the system to behave correctly.
> >>>
> >> My memory is that Nick's work just didn't have the momentum to get
> >> pushed in. It all seemed very reasonable though, I think our hatred of
> >> buffered heads just wasn't yet bigger than the fear of moving away.
> >>
> >> But, the bigger question is how big are the blocks going to be? At some
> >> point (64K?) we might as well just make a log structured dm target and
> >> have a single setup for both shingled and large sector drives.
> > There is no real point. Even with 4k drives today using 4k sectors in
> > the filesystem, we still get 512 byte writes because of journalling and
> > the buffer cache.
> I think that you are wrong here James. Even with 512 byte drives, the IO's we
> send down tend to be 4k or larger. Do you have traces that show this and details?

It's mostly an ext3 journalling issue ... and it's only metadata and
mostly the ioschedulers can elevate it into 4k chunks, so yes, most of
our writes are 4k+, so this is a red herring, yes.

> >
> > The question is what would we need to do to support these devices and
> > the answer is "try to send IO in x byte multiples x byte aligned" this
> > really becomes an ioscheduler problem, not a supporting large page
> > problem.
> >
> > James
> >
> Not that simple.
> The requirement of some of these devices are that you *never* send down a
> partial write or an unaligned write.

But this is the million dollar question. That was originally going to
be the requirement of the 4k sector devices but look what happened in
the market.

> Also keep in mind that larger block sizes allow us to track larger
> files with
> smaller amounts of metadata which is a second win.

Larger file block sizes are completely independent from larger device
block sizes (we can have 16k file block sizes on 4k or even 512b
devices). The questions on larger block size devices are twofold:

1. If manufacturers tell us that they'll only support I/O on the
physical sector size, do we believe them, given that they said
this before on 4k and then backed down. All the logical vs
physical sector stuff is now in T10 standards, why would they
try to go all physical again, especially as they've now all
written firmware that does the necessary RMW?
2. If we agree they'll do RMW in Firmware again, what do we have to
do to take advantage of larger sector sizes beyond what we
currently do in alignment and chunking? There may still be
issues in FS journal and data layouts.


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