On 15.03.2022 14:14, Johannes Thumshirn wrote:
On 15/03/2022 14:52, Javier González wrote:
On 15.03.2022 14:30, Christoph Hellwig wrote:
On Tue, Mar 15, 2022 at 02:26:11PM +0100, Javier González wrote:
but we do not see a usage for ZNS in F2FS, as it is a mobile
file-system. As other interfaces arrive, this work will become natural.
ZoneFS and butrfs are good targets for ZNS and these we can do. I would
still do the work in phases to make sure we have enough early feedback
from the community.
Since this thread has been very active, I will wait some time for
Christoph and others to catch up before we start sending code.
Can someone summarize where we stand? Between the lack of quoting
from hell and overly long lines from corporate mail clients I've
mostly stopped reading this thread because it takes too much effort
actually extract the information.
Let me give it a try:
- PO2 emulation in NVMe is a no-go. Drop this.
- The arguments against supporting PO2 are:
- It makes ZNS depart from a SMR assumption of PO2 zone sizes. This
can create confusion for users of both SMR and ZNS
- Existing applications assume PO2 zone sizes, and probably do
optimizations for these. These applications, if wanting to use
ZNS will have to change the calculations
- There is a fear for performance regressions.
- It adds more work to you and other maintainers
- The arguments in favour of PO2 are:
- Unmapped LBAs create holes that applications need to deal with.
This affects mapping and performance due to splits. Bo explained
this in a thread from Bytedance's perspective. I explained in an
answer to Matias how we are not letting zones transition to
offline in order to simplify the host stack. Not sure if this is
something we want to bring to NVMe.
- As ZNS adds more features and other protocols add support for
zoned devices we will have more use-cases for the zoned block
device. We will have to deal with these fragmentation at some
point.
- This is used in production workloads in Linux hosts. I would
advocate for this not being off-tree as it will be a headache for
all in the future.
- If you agree that removing PO2 is an option, we can do the following:
- Remove the constraint in the block layer and add ZoneFS support
in a first patch.
- Add btrfs support in a later patch
(+ linux-btrfs )
Please also make sure to support btrfs and not only throw some patches
over the fence. Zoned device support in btrfs is complex enough and has
quite some special casing vs regular btrfs, which we're working on getting
rid of. So having non-power-of-2 zone size, would also mean having NPO2
block-groups (and thus block-groups not aligned to the stripe size).
Thanks for mentioning this Johannes. If we say we will work with you in
supporting btrfs properly, we will.
I believe you have seen already a couple of patches fixing things for
zone support in btrfs in the last weeks.
Just thinking of this and knowing I need to support it gives me a
headache.
I hope we have help you with that. butrfs has no alignment to PO2
natively, so I am confident we can find a good solution.
Also please consult the rest of the btrfs developers for thoughts on this.
After all btrfs has full zoned support (including ZNS, not saying it's
perfect) and is also the default FS for at least two Linux distributions.
Of course. We will work with you and other btrfs developers. Luis is
helping making sure that we have good tests for linux-next. This is in
part how we have found the problems with Append, which should be fixed
now.
Thanks a lot,
Johannes
