NVMe ZNS Support Coming To Linux 5.9

[oiaohm]

Any idea what it'll take for ZFS to support it?

The reality is a hard one. Current ZFS developers are not going to get early access to ZNS hardware. They are going to have to wait for ZNS hardware to be on the open market or change their licensing to something hardware vendors agree with. CDDL is not a good license choice.

Also thinking ZFS developer were given heads up about SMR drives

https://openzfs.org/w/images/2/2a/Host-Aware_SMR-Tim_Feldman.pdf

Yes this is 2014 and ZFS currently does not have a roadmap even to support SMR yet. Same changes to support SMR drives has to happen to support ZNS drives.\

Remember we are over 6 years in on this zoned storage problem. ZFS has not started the work. File systems like XFS started 5 years ago working on fixing up items for zoned storage and are not ready yet. Yes if ZFS developers start now it could be over a decade before they have something ready.

Will EXT4 et al need to re-create their volumes in order to use ZNS - or will it work with existing volumes so long as the kernel supports it?

And

Hi @oiaohm, thank you for that elaborate explanation!
I think it makes a bit more sense to me now.

Note for the "developer" point of view, i meant "just" a developer. Specifically not a filesystem dev
But it looks like we, developers, don't have to care about this at all.

Lastly, would this zone stuff be auto-enabled on - say - ext4 once all pieces are in place? Or is this going to be something that's off by default where users (or distributions) can opt-in to enable it?

Both of you have the same question over ext4. Ext4 is fairly much screwed on SMR and ZNS going forwards.

https://zonedstorage.io/linux/fs/

However, support for host managed zoned block devices is not provided as some fundamental ext4 design aspects cannot be easily changed to match host managed device constraints.
Yes Ext4 will be option on ZNS and SMR drives but ext4 will be sitting on top of dm-zoned so you still have lot of the problems of device managed SMR or existing SSD plus now extra CPU overhead as well. Of course there is still work altering ext4 operations to play better with dm-zoned being under it.

Its the F2FS, BTRFS and XFS that are working on full Zoned Block Devices support. Mind you in place conversion from ext2-4 to BTRFS is possible explains lot of interest in btrfs performance.

Please note XFS is taking a while to get perfectly done for zoned block devices as well as it requiring on disc changes.

Its also not exactly auto enabled if you have a pure host managed SMR HDD or ZNS SSD you will not be able to use those devices unless you have support for the zoned storage tech. Host aware versions of SMR HDD and ZNS SSD it might be possible to auto enable after the fact. We are not sure if host aware ZNS SSD will even exist. ZNS could be a pure choice between device managed SSD as we are use to and host managed ZNS SSD with nothing in the middle. Of course the ZNS SSD could be cheap and perform well if you have software that supports it due to not requiring the DRAM device managed SSD does.

Distributions deciding not to support ZNS will also not be supporting SMR HDD properly either. Yes that is another party I did not mention Distributions will have to update their install process to support Zoned blocked devices. Us installing and setting up drives will have to get use to a few slightly different processes.

[deusexmachina]

oiaohm Good info... I wonder if there are any other alternatives for ZFS to catch up. I don't know much about why ZNS is better - but I guess it is needed on top of TRIM. You read the ZFS dev mailing list or something? I wish more stuff like this was discussed on forums, but it seems most devs like the mailing list style discussion - but I always find it tedious.

[markg85]

Oh, now it hits me. The ZNS stuff is a delicate dance between hardware changes and filesystems. Thus far i thought the gist of it was the driver for those SSD's would "just" expose some more functionality that filesystems "could" use. But from what you've told it looks to be a combination of must-haves. In other words, a ZNS enabled device won't work on a filesystem that doesn't support it.

[oiaohm]

oiaohm Good info... I wonder if there are any other alternatives for ZFS to catch up. I don't know much about why ZNS is better - but I guess it is needed on top of TRIM. You read the ZFS dev mailing list or something? I wish more stuff like this was discussed on forums, but it seems most devs like the mailing list style discussion - but I always find it tedious.

There is no easy way to catch up. ZNS is zoned based storage like SMR in harddrives. Low level file system operation changes required these take lots of work and lots of time to validate you have done them right. This is a case that you should be seeing changes appearing in the file system update change logs saying X changes done to prep for either zoned based storage devices or SMR drives and ZFS at this stage has not started doing the ground work.

Biggest reasons for ZNS route is improving the wear pattern on SSD devices to extend their operational life as well as improving performance by reducing stall events. It also comes with cost savings and reduction in SSD controller complexity requirements. Basically who does not want cheaper SSD that perform well of course ZNS comes with a trade off that your OS file system and block device system need to be smarter.

Oh, now it hits me. The ZNS stuff is a delicate dance between hardware changes and filesystems. Thus far i thought the gist of it was the driver for those SSD's would "just" expose some more functionality that filesystems "could" use. But from what you've told it looks to be a combination of must-haves. In other words, a ZNS enabled device won't work on a filesystem that doesn't support it.

We will not know until vendors start releasing ZNS hardware.

SMR drives on the market come in 3 forms.
1) Device managed as in pretend to be old style drives hide the different in the controller.
2) Host aware this is a device that can pretend to be a old style drive also take instructions to run SMR propery.
3) Host managed this will not work unless the file system/block system support it in the OS.

ZNS when you look at it. All existing SSD are operating in device managed so device managed is already on the market.
We have dramless SSD on the market. https://journals.plos.org/plosone/ar...l.pone.0229645 and we will be getting the HMB form that uses host memory instead of DRAM chips in the SSD device.

So there are 3 existing forms of SSD in the device managed.
SSD with DRAM in the device.
SSD without DRAM in the device and attempts to make do and performs really badly.
SSD without DRAM using HMB this does not in fact catch up to SSD with DRAM in perform in a lot of cases.

This means there can be 3 forms in the host aware if vendors decide to make them for SSD.

Now a ZNS based device will most likely to be DRAM less as removing DRAM saves a lot of cost. Host Aware is a option for those designing the ZNS controller but it means more silicon in the controller so more cost. So host managed ZNS will also be on the market as the cheapest SSD option.

Linux using dm-zoned can technically put any file system on a zoned storage device be it SMR/ZNS but technically being able todo it does not mean it will perform well. If you care about performance on ZNS or SMR drives you really need the file system to support zoned storage properly so you get the performance on the table.

[Azrael5]

Is ZNS a feature to match any kind of file system?

[oiaohm]

Is ZNS a feature to insert into any kind of file system?

That the catch. Not all file system design are suitable to make zoned device effective.

[WorBlux]

Is ZNS a feature to insert into any kind of file system?

Traditional file systems have problems. CoW or log-structured file systems are easier to adapt.

Some applications can skip the filesystem entirely as well using a library to turn raw zns/zbc into a key/object store.