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

Rainer Krienke krienke at uni-koblenz.de
Fri Apr 3 12:54:36 CEST 2020


Hello Alwin,

thanks for you very much answer.

Regarding LVM:
I initially thought the cache size is 25MB for each RBD device. So a LVM
based on in my case 4 RBD devices could loose 100MB. This would have
been a difference. But after reading the docu again it seems that this
cache is for all rbds of one ceph client (pve host).  Then you are right
and it does not  make a difference.

Thanks
Rainer

Am 03.04.20 um 10:44 schrieb Alwin Antreich:
> 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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Rainer Krienke, Uni Koblenz, Rechenzentrum, A22, Universitaetsstrasse  1
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