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

Rainer Krienke krienke at uni-koblenz.de
Fri Apr 3 10:00:58 CEST 2020


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.

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?

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?

Thanks a lot for your help
Rainer Krienke, Uni Koblenz, Rechenzentrum, A22, Universitaetsstrasse  1
56070 Koblenz, Web: http://www.uni-koblenz.de/~krienke, Tel: +49261287 1312
PGP: http://www.uni-koblenz.de/~krienke/mypgp.html,     Fax: +49261287

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