Re: The chunk size paradox

[Phillip Susi <psusi@ubuntu.com>]

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On 01/02/2014 06:16 PM, Stan Hoeppner wrote:
> Thank you for this information.  Now, if I actually had a 4K drive
> in my hands, and plugged it in, directly formatted it with XFS, no
> partitions, would the LBA addressing be 4K or 512B?  Or would I
> need to tweak kernel parameters?  Or possibly rebuild my kernel to
> support 4K sectors?

Addressing would be in 4k sectors, no need for tweaking.  You can see
it in action with the scsi_debug module if you don't have the actual
hardware.

> It's not relevant because you don't create an md RAID set from
> CD-ROM.

Well the context wasn't specifically for md; you claimed linux did not
support non 512 byte sectors full stop.  And you *can* build an md
raid on top of cd/dvd-rw, though obviously that's a little nutty.

> IIRC, there was a lengthy discussion about this on mm back when
> some folks wanted to use 16K-4GB pages on Itanium, and later 2M
> pages on x86-64, to cut down on the amount of memory required for
> page tables and to increase performance for big memory workloads.
> As I recall the arguments for continuing to use 4K pages across the
> world of Linux, regardless of architecture capability, and to NOT
> make it configurable as in HP-UX, were, paraphrasing:

Some archs don't support 4k pages at all.

> 1.  The kernel manipulates "everything" in pages so we need
> consistency 2.  While larger pages saves page table space and
> increases throughput for large memory intensive workloads, it
> causes more waste in other structures and increases bandwidth
> demands for data that are smaller than the page size
> 
> So, IIRC, it was decided that the page size would remain 4K
> basically forever.  So while it is *technically* possible to have a
> larger page size in Linux, it is absolutely not supported by the
> kernel team, nor any distro kernel, AFAIK.

Yep, that's why the default on those archs that have a choice is still
4k, but some do give the option for larger.  If you scan the ext4 and
md mailing lists you should find a few discussions of people doing
this, or just using larger block sizes on archs that only support
larger page sizes.

> And I'd guess a whole host of things will likely break as a result
> if you don't correctly modify much of the kernel source before
> running make.  See above.

Nope; otherwise it wouldn't be a Kconfig option.  Remember, the kernel
had to grow support for non 4k pages to work at all on archs that
don't support them.

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