[PVE-User] Need some advice on pve writeback caching

Alwin Antreich a.antreich at proxmox.com
Fri Apr 3 10:44:47 CEST 2020


Hello Rainer,

On Fri, Apr 03, 2020 at 10:00:58AM +0200, Rainer Krienke wrote:
> Hello,
> 
> I played around with rbd caching by activating "Writeback" mode in
> proxmox6. This really helps for write performance so I would like to use
> it but the documenattion says that a possible danger is a power outage.
The default cache size is 25 MiB. That data might be lost, but the image
should still be consistent.

> 
> Now I have a battery backup but you never know if the outage is just a
> minute or an hour, and batteries would not keep the servers up for more
> that say 15min.
> 
> So what I thought about is to enable rbd caching but when a power outage
> occurs I would like to automatically disable it. Using proxmox this
> requires a reboot of the VM so this is no option. But perhaps I could
> set globally a ceph variable eg "rbd cache size" to "0"  and thus
> effectively would disable writeback caching. Should this work?
These settings only take effect after a VM was cold booted. Wouldn't it
just be better to shutdown the VMs, once the UPS notices the power
outage?

> 
> Another question about "Wrtiteback" rbd caching is if I should also
> enable it for disks with a lvm striped over 4 rbd images?  Each striped
> LVs has a filesystem on top. Is it also advisable to use rbd caching in
> this scenario as it is for a single disk?
Why should it differ?

--
Cheers,
Alwin



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