[linux-lvm] Reserve space for specific thin logical volumes

Gionatan Danti g.danti at assyoma.it
Tue Sep 12 21:36:45 UTC 2017

Il 12-09-2017 21:44 matthew patton ha scritto:
>>  Wrong: Qemu/KVM *does* honors write barrier, unless you use  
>> "cache=unsafe".
> seems the default is now 'unsafe'.
> http://libvirt.org/formatdomain.html#elementsDisks

The default is cache=none

> Only if the barrier frame gets passed along by KVM and only if you're
> running in "directsync" (or perhaps 'none?') mode there is no
> guarantee any of it hit the platter. Let's assume a hypervisor I/O is
> ahead of VM 'A's barrier frame, and blows up the thinLV. Then yes it's
> possible to propagate the ENOSPACE or other error back to the VM 'A'
> to realize that write didn't actually succeed. The rapid failure
> cascade across resident VMs is not going to end well either.

fsynched writes that hits a full pool returns EIO to the upper layer

> But if that's the only condition it works under they why bother with
> XFS on top of thin? You already mentioned that XFS is lousy about
> actually detecting underlying block layer problems until much too
> late. Just provision the Thin LV and map it directly to the VM. If
> your choice of hypervisor is broken, fix it, or choose another that
> doesn't have the problem. But really, using ThinLV for guest VM and
> asking the hypervisor to not blow up is nuts. Buy a friggin disk. Or
> put the onus on try/error where it belongs - inside the guest VM. Why
> is that so hard?

KVM is the most valid GPL hypervisor, and libvirt is the virtualization 
library of choice.
But I can not fix/implement thin pool/volumes management alone.

> You're a web-hosting company and you're trying to duck the laws of
> economics and the reality of running a business where other often
> clueless people trust you to keep their data intact?

Please, don't elaborate on things you don't know.
I asked a specific question on the linux-lvm list, and (as always) I 
learnt something. I don't see any problem in doing that.

> Given the screwball way you're going about handling your customer
> data, why are you trying to be 'creative'? Storage is your MOST
> important and vital capability and naturally your most expensive. Get
> real and spend the money. Storage is where you NEVER cut corners and
> you NEVER attempt naive optimization efforts.
> I'm not quibbling over XFS, you could have picked EXT4 for all I care.
> The point is you're trying to get jiggy with customer data to save
> pennies. We have a saying, picking up pennies in front of a road
> paver. If you're selling capacity you don't have, it's not only
> dishonest, but there are proven and well understood ways to do so (eg.
> NFS or thin-alloc iSCSI) aimed at a proper storage head that is
> properly managed. But ultimately you HAVE to be able to satisfy the
> instant demands of all users even if that means they all suddenly wake
> up and want to use what you supposedly sold them and entered into a
> contract to supply by taking  their money.

Again, please don't speak about things you don't know.
I am *not* interested in thin provisioning itself at all; on the other 
side, I find CoW and fast snapshots very useful.

> Computing/Storage is not a ponzi scheme.

Thanks for remind me that.

Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
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