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 > > _______________________________________________ > pve-user mailing list > email@example.com > https://pve.proxmox.com/cgi-bin/mailman/listinfo/pve-user > -- 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 1001312 _______________________________________________ pve-user mailing list firstname.lastname@example.org https://pve.proxmox.com/cgi-bin/mailman/listinfo/pve-user