[PATCH] storage: only fallocate when allocation matches capacity

Christian Ehrhardt christian.ehrhardt at canonical.com
Thu Sep 3 13:37:02 UTC 2020

On Thu, Sep 3, 2020 at 12:49 PM Richard Laager <rlaager at wiktel.com> wrote:
> On 9/3/20 5:18 AM, Christian Ehrhardt wrote:
> > Even if my fix lands, we are back to square one and would need
> > virt-manager to submit a different XML.
> > Remember: my target here would be to come back to pralloca=metadata as
> > it was before for image creations from virt-manager.
> Why is that your goal?
> If this is simply because OpenZFS doesn't support fallocate(mode=0),

Yeah it was because behavior changed for users on upgrades and became
slow (if on ZFS).
And since the symptom was restricted to just that and also just a
slowdown the prio was always low anyway.

> that has (finally!) been resolved for the next release:
> https://github.com/openzfs/zfs/commit/f734301d2267cbb33eaffbca195fc93f1dae7b74

Glad to hear that - and I agree that fallocate @ COW/Compression FS is
not really applicable.

So it seems "my need" for this is completely gone once we get the change above.
Thanks Richard for the FYI on it!

> ZFS will "fake" the fallocate() request. It'll check to make sure
> there's enough free space at the moment, which is about all it can do
> anyway. It can't reserve the space anyway, mostly because it is a
> copy-on-write filesystem. Even if the application writes zeros, ZFS will
> just throw them away anyway (assuming you are using compression, which
> everyone should be).
> > On the libvirt side allocation>capacity sounds like being wrong anyway.
> > And if that is so we have these possible conditions:
> > - capacity==allocation now and before my change falloc
> > - capacity>allocation now and before my change metadata
> > - capacity<allocation before my change falloc, afterwards metadata
> > (but this one seems invalid anyway)
> >
> > So I wonder are we really back at me asking Cole to let virt-manager
> > request things differently which is how this started about a year ago?
> Setting aside cases of semi-allocation (capacity > allocation != 0) and
> overprovisioning (allocation > capacity), I assume the common cases are
> thin provisioning (allocation == 0) and thick provisioning (capacity ==
> allocation).
> virt-manager (at least in the way I use it) asks explicitly for the
> allocation and capacity. If virt-manager is properly conveying (and I'd
> assume it is) the user's capacity and allocation choices from the GUI to
> libvirt, then virt-manager is working correctly in my view and should be
> left alone.
> I believe the main goal for thick provisioning is to reserve the space
> as best as possible, because ENOSPC underneath a virtual machine is bad.
> Secondary goals would be allocating the space relatively contiguously
> for performance and accounting for the space immediately to help the
> administrator keep track of usage.
> If the filesystem supports fallocate(), using it accomplishes all of
> these goals in a very performant way. If the filesystem does not support
> fallocate(), then the application can either write zeros or do nothing.
> Writing zeros is slow, but achieves the goals to the extent possible.
> Not writing zeros is fast, but does not reserve/account for the space;
> though, depending on the filesystem, that might not be possible anyway.
> I think the question fundamentally comes down to: how strong do you take
> a "thick provisioning" request? Do you do everything in your power to
> achieve it (which would mean writing zeros*) or do you treat it as a
> hint that you'll only follow if it is fast to do so?
> If it's a demand, then try fallocate() but fall back to writing zeroes.
> (glibc's posix_fallocate() does exactly this.). If it's a hint, then
> only ever call fallocate().
> I think it is reasonable to treat it as a demand and write zeros if
> fallocate() fails. If it is too slow, the admin will notice and can make
> the decision to (in the future) stop requesting thick provisioning and
> just request thin provisioning.
> In the ZFS case, why is the admin requesting thick provisioning anyway?
> * One could go further and defeat compression by writing random data.
>   But that seems extreme, so I'm going to ignore that.
> --
> Richard

Christian Ehrhardt
Staff Engineer, Ubuntu Server
Canonical Ltd

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