Re: [PATCH] nvme: honor the volatile write cache enable state

[Erwan Velu]

Ok let me clarify this.

This disk is configured by default with VWC_PRESENT enabled, but the
Volatile Write Cache is disabled (NVME_FEAT_VWC_ENABLE=0), so the
device has a Volatile Write Cache, but it's disabled.
In that configuration, even though VWC_PRESENT is set to 1 and the
volatile cache is disabled, the kernel treats it as enabled and
switches to "write back" mode, causing tons of sync and bad
performance.

In this table, with my patch, I've enabled and disabled the volatile
write cache using nvme set-feature -f 6; this only affects
NVME_FEAT_VWC_ENABLE, and VWC_PRESENT is still reported as present.
If the volatile cache is enabled, write-back mode engages, resulting
in poor performance.
If the volatile cache is disabled, write-through mode engages with the
expected performance.

Note that in the current mainline state, my drive is _always_ in
write-back mode because NVME_FEAT_VWC_ENABLE isn't being considered,
resulting in horrible performance.
That's why this patch ensures the volatile write cache status (which
the kernel uses to decide which caching mode to engage) depends on
both VWC_PRESENT __and__ NVME_FEAT_VWC_ENABLE.

Hope that's make it more clear,
Erwan,

Le ven. 16 oct. 2026 à 19:27, Keith Busch <kbusch@xxxxxxxxxx> a écrit :
>
> On Fri, Oct 16, 2026 at 07:15:20PM +0200, Erwan Velu wrote:
> > Le ven. 16 oct. 2026 à 18:57, Keith Busch <kbusch@xxxxxxxxxx> a écrit :
> > >
> > > On Fri, Oct 16, 2026 at 04:12:08PM +0200, Erwan Velu wrote:
> > > >   job                   | cache   |   IOPS | fsync avg | flushes |  gain
> > > >   ----------------------+---------+--------+-----------+---------+------
> > > >   psync buffered, QD1   | back    |   3885 |   247.6us |  116552 |
> > > >                         | through |  48325 |    15.7us |       0 | x12.4
> > > >   ----------------------+---------+--------+-----------+---------+------
> > > >   io_uring direct, QD1  | back    |   3886 |   238.2us |  116593 |
> > > >                         | through |  41924 |     5.7us |       0 | x10.8
> > > >   ----------------------+---------+--------+-----------+---------+------
> > > >   io_uring direct, QD32 | back    |   5088 |  6173.5us |  117793 |
> > > >                         | through | 140196 |   107.0us |       0 | x27.6
> > > >
> > > > From NVMe 1.4c (sections 5.21.1.6 and 6.8) up to the NVMe Base
> > > > Specification 2.4 (sections 5.2.30.1.4 and 7.2), a Flush command has no
> > > > effect if the volatile write cache is not enabled.
> > >
> > > Yeah, the NVMe Flush operation should be a no-op in the device, so the
> > > dramatic performance difference is a bit alarming. Surely the block
> > > stack's flush machinery is not the reason for the difference, right?
> > >
> > > If the device's flush handling is this bad when VWC is disabled, what
> > > happens when you have it enabled? Can you re-run the same test with that
> > > setting?
> > >
> > > I'm not necessarily against what you're proposing here, but your numbers
> > > are unexpected.
> >
> > Maybe my table here isn't probably clear enough.
> > The numbers shown in the  "write back" mode were done with VWC enabled.
>
> But you said the table came from tests "with WCE cleared" in the VWC
> feature. ?