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

From: Ric Wheeler
Date: Wed Jan 22 2014 - 09:59:03 EST

On 01/22/2014 09:34 AM, Mel Gorman wrote:
On Wed, Jan 22, 2014 at 09:10:48AM -0500, Ric Wheeler wrote:
On 01/22/2014 04:34 AM, 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

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.

I have a somewhat hazy memory of Andrew warning us that touching
this code takes us into dark and scary places.

That is a light summary. As Andrew tends to reject patches with poor
documentation in case we forget the details in 6 months, I'm going to guess
that he does not remember the details of a discussion from 7ish years ago.
This is where Andrew swoops in with a dazzling display of his eidetic
memory just to prove me wrong.

Ric, are there any storage vendor that is pushing for this right now?
Is someone working on this right now or planning to? If they are, have they
looked into the history of fsblock (Nick) and large block support (Christoph)
to see if they are candidates for forward porting or reimplementation?
I ask because without that person there is a risk that the discussion
will go as follows

Topic leader: Does anyone have an objection to supporting larger block
sizes than the page size?
Room: Send patches and we'll talk.

I will have to see if I can get a storage vendor to make a public statement, but there are vendors hoping to see this land in Linux in the next few years. I assume that anyone with a shipping device will have to at least emulate the 4KB sector size for years to come, but that there might be a significant performance win for platforms that can do a larger block.

Note that windows seems to suffer from the exact same limitation, so we are not alone here with the vm page size/fs block size entanglement....


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