Re: dm-writecache issue

[Eric Sandeen <sandeen@sandeen.net>]

On 9/18/18 9:29 AM, Mikulas Patocka wrote:

> On Tue, 18 Sep 2018, Eric Sandeen wrote:

...

>> See also 
>> https://www.intel.com/content/www/us/en/support/articles/000006392/memory-and-storage.html
>>
>> -Eric
> 
> And does it really support native 512-byte writes? Or does it emulate 
> 512-byte writes by doing read-modify-write? That needs to be benchmarked, 
> the paper doesn't say that.

Interesting from a manual tuning perspective, but not from a default
behavior perspective.

I'm just pointing out that Intel does seem to give the user a choice about
the /advertised/ geometry for some of their SSDs.

> Memory is expensive and reducing SSD sector size increases memory 
> requirement on the SSD. I doubt that any SSD vendor would want to use 
> 8-times more memory just to support 512-byte sectors natively.

Marketing decisions aside, we just can't safely ignore what the device
tells us about these IO sizes.

We have similar issues with raid devices which report nonsensical
optimal and mininum IO sizes.  If it's reporting bad info, the user
can override it, but they have to be sure to get it right.  For default
behavior, mkfs.xfs has no choice but to use what the device tells it
to use.

-Eric