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

[NeilBrown <neilb@suse.com>]

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?
> 
> Cheers,
> Jes
> 
> 
> Jes Sorensen (1):
>   raid0: Disable discard per default due to performance uncertainty
> 
>  drivers/md/raid0.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 

RAID1?  RAID0??  I hate it when I do that!

Doesn't the scheduler merge adjacent discard requests?

Or is this some non-SATA/SCSI SSD that has a 'make_request_fn' driver?
I think I came across one of those before (NVMe).
In that case - the driver needs to be fixed.

NeilBrown