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

James Bottomley
2014-01-22T19:30:19Z

On Wed, 2014-01-22 at 13:39 -0500, Ric Wheeler wrote:
> On 01/22/2014 01:35 PM, James Bottomley wrote:
> > On Wed, 2014-01-22 at 13:17 -0500, Ric Wheeler wrote:
[...]
> >> I think that the key to having the file system work with larger
> >> sectors is to
> >> create them properly aligned and use the actual, native sector size as
> >> their FS
> >> block size. Which is pretty much back the original challenge.
> > Only if you think laying out stuff requires block size changes. If a 4k
> > block filesystem's allocation algorithm tried to allocate on a 16k
> > boundary for instance, that gets us a lot of the performance without
> > needing a lot of alteration.
>
> The key here is that we cannot assume that writes happen only during
> allocation/append mode.

But that doesn't matter at all, does it? If the file is sector aligned,
then the write is aligned. If the write is short on a large block fs,
well we'd just have to do the RMW in the OS anyway ... is that any
better than doing it in the device?

> Unless the block size enforces it, we will have non-aligned, small
> block IO done
> to allocated regions that won't get coalesced.

We always get that if it's the use pattern ... the question merely
becomes who bears the burden of RMW.

> > It's not even obvious that an ignorant 4k layout is going to be so
> > bad ... the RMW occurs only at the ends of the transfers, not in the
> > middle. If we say 16k physical block and average 128k transfers,
> > probabalistically we misalign on 6 out of 31 sectors (or 19% of the
> > time). We can make that better by increasing the transfer size (it
> > comes down to 10% for 256k transfers.
>
> This really depends on the nature of the device. Some devices could
> produce very
> erratic performance

Yes, we get that today with misaligned writes to the 4k devices.

> or even (not today, but some day) reject the IO.

I really doubt this. All 4k drives today do RMW ... I don't see that
changing any time soon.

> >> Teaching each and every file system to be aligned at the storage
> >> granularity/minimum IO size when that is larger than the physical
> >> sector size is
> >> harder I think.
> > But you're making assumptions about needing larger block sizes. I'm
> > asking what can we do with what we currently have? Increasing the
> > transfer size is a way of mitigating the problem with no FS support
> > whatever. Adding alignment to the FS layout algorithm is another. When
> > you've done both of those, I think you're already at the 99% aligned
> > case, which is "do we need to bother any more" territory for me.
> >
>
> I would say no, we will eventually need larger file system block sizes.
>
> Tuning and getting 95% (98%?) of the way there with alignment and IO
> scheduler
> does help a lot. That is what we do today and it is important when
> looking for
> high performance.
>
> However, this is more of a short term work around for a lack of a
> fundamental
> ability to do the right sized file system block for a specific class
> of device.
> As such, not a crisis that must be solved today, but rather something
> that I
> think is definitely worth looking at so we can figure this out over
> the next
> year or so.

But this, I think, is the fundamental point for debate. If we can pull
alignment and other tricks to solve 99% of the problem is there a need
for radical VM surgery? Is there anything coming down the pipe in the
future that may move the devices ahead of the tricks?

James