Re: Feature Request: Device Manager Fake Trim / Zero Trim

[charlesfdotz@tutanota.com]

Oct 9, 2023, 09:43 by mpatocka@redhat.com:

>
>
> On Mon, 9 Oct 2023, Hannes Reinecke wrote:
>
>> On 10/9/23 02:56, charlesfdotz@tutanota.com wrote:
>> > Hello,
>> >
>> > I would like to request a new device manager layer be added that accepts
>> > trim requests for sectors and instead
>> > writes zeros to those sectors.
>> >
>> > This would be useful to deal with SMR (shingled magnetic recording) drives
>> > that do not support trim. Currently
>> > after an SMR drive has had enough data written to it the performance drops
>> > dramatically because the disk must
>> > shuffle around data as if it were full and without trim support there is no
>> > way to inform the disk which sectors
>> > are no longer used. Currently there's no way to "fix" or reset this without
>> > doing an ATA secure erase despite
>> > many of these disk being sold without informing customers that they were SMR
>> > drives (western digital was sued
>> > for selling SMR drives as NAS drives).
>> >
>> Gosh, no, please don't.
>> SMR drives have a write pointer, and if the zone needs to be reset you just
>> reset the write pointer. Writing zeroes will result in the opposite; the zone
>> continues to be full, and no writes can happen there.
>>
>> Which drive is this?
>>
>> Cheers,
>>
>> Hannes
>>
The drive in the link is a ST8000DM004-2CX188 according to the poster. To be clear I'm talking about device managed SMR drives that don't support trim and do not report being SMR *not* host aware drives. The DM-SMR drives work fine until you've written their total capacity once then the write performance craters permanently because the drive thinks all the data on it is important and has to reshuffle it constantly during writes. We are talking sub-1MiB/s and hangs waiting on the controller to move things around that can be several minutes. WD was recently sued for selling some of these as "WD Red NAS" drives.

I'm not sure why writing zeros appears to work so this just speculation on my part but I was thinking the drive might be smart enough to mark sectors full of zeros as unused and then return any request for an unused sector as zeros. 
>
> BTW. what about converting trims to zone reset? If the trim spans a whole
> zone, the kernel could convert it to REQ_OP_ZONE_RESET on host-aware zoned
> devices.
>
> Mikulas
>
This particular request was to try and make (some) device managed SMR drives that don't report to the host that they are SMR more usable. I was thinking if they are a model where zero sectors appear to "trim" one could just put this "fake-trim" device mapper layer over it and then it would act like a device managed SMR drive that supports trim.

Sincerely,
Chuck