[U-Boot] [PATCH] [RFC] blk: Increase cache element size

[Marek Vasut]

On 08/15/2018 06:12 PM, Tom Rini wrote:
> On Wed, Aug 15, 2018 at 06:04:50PM +0200, Marek Vasut wrote:
> 
>> On 08/15/2018 04:30 PM, Tom Rini wrote:
>>> On Wed, Aug 08, 2018 at 01:20:29PM +0200, Marek Vasut wrote:
>>>
>>>> Cache up to 4 kiB entries. 4 kiB is the default block size on ext4, yet
>>>> the underlying block layer devices usually report support for 512B . In
>>>> most cases, the 512B support is emulated (ie. SD cards, SSDs, USB sticks
>>>> etc.) and the real block size of those devices is much bigger.
>>>>
>>>> To avoid performance degradation with such devices and FS setup, bump
>>>> the maximum cache entry size to 4 kiB.
>>>>
>>>> Signed-off-by: Marek Vasut <marex at denx.de>
>>>> Cc: Tom Rini <trini at konsulko.com>
>>>> Cc: Simon Glass <sjg at chromium.org>
>>>
>>> Reviewed-by: Tom Rini <trini at konsulko.com>
>>>
>>> I'll pick this up post v2018.09 if no one objects, thanks!
>>
>> I object. I was hoping there'd be some discussion on how to solve this
>> in a future-proof manner ... it's only a matter of time until someone
>> uses ext4 with 8k blocks on an SSD ...
> 
> In general, sure?  In specific, mkfs.ext4 1.42.13 man page says 1/2/4KiB
> are the only valid values of block size, and based on having to whack
> this for some other projects it's pretty common for OpenEmbedded at
> least to spit out 1KiB block size images.

OE spits 4k , that's how I triggered it,
meta/classes/image_types.bbclass:EXTRA_IMAGECMD_ext2 ?= "-i 4096"
meta/classes/image_types.bbclass:EXTRA_IMAGECMD_ext3 ?= "-i 4096"
meta/classes/image_types.bbclass:EXTRA_IMAGECMD_ext4 ?= "-i 4096"

> So unless you know of cases
> today (or tomorrow, but not next year) where 8KiB is common or likely,
> we should probably just bump this for now and maybe make it a tunable so
> it's easily changed?

It is already tunable, see blkcache config in blkcache command.

But what I'd like to see is somehow the FS and the underlying storage
negotiating the best settings. Can we get the FS block size from the
block cache perspective ?

-- 
Best regards,
Marek Vasut