Re: [PATCH 0/1] RFC: Unreliable discard performance can cripple RAID1

[Jes Sorensen <Jes.Sorensen@redhat.com>]

Roman Mamedov <rm@romanrm.net> writes:
> On Tue, 23 Jun 2015 20:26:12 -0400
> Jes.Sorensen@redhat.com wrote:
>
>> From: Jes Sorensen <Jes.Sorensen@redhat.com>
>> 
>> Neil,
>> 
>> I have been hitting issues with discard being ridiculously slow on
>> arrays with certain typs of SSDs that seem to serialize discard
>> processing.
>> 
>> This is particularly bad as I have seen systems where the IMSM BIOS
>> defaults to 4KB chunk size, combined with these badly performing
>> drives, it could bump the mkfs on an array from seconds to over 40
>> minutes. Most users will stick to the defaults and then hit the
>> problem during install without understanding why it goes wrong :(
>> 
>> The problem is that there is no way to benchmark our way to this or
>> somehow test if a drive performs discard at reasonable speed. I
>> suggest we take an approach similar to that of RAID456 and default to
>> disabling discard, except for the case where the user knows the drives
>> are safe.
>> 
>> Thoughts?
>
> It's very unfortunate if you would cripple all the good SSD models because of
> a few bad ones. No one will remember to explicitly put the override to enable
> TRIM, or perhaps even know that it gets disabled in md in the first place. The
> only thing they will later notice is lowered performance and lifespan of their
> SSDs.

We already disable discard per default on raid456 in a similar manner
because some of them unreliably reports discard_zeroes_data when they
in reality don't.

If there was a way to reliably detect these things it would be fine,
unfortunately there isn't.

Jes