Well when there's no power, it isn't flushed and the ZFS write transaction isn't complete, end of story. ZFS is still in a consistent state but doesn't have any transactions relating to the last flush that didn't happen. Same idea as a database.Sebulon said:@AndyUKG
And the operative word in that sentence is "should" I mean, what good does a cache flush command do when thereÂ´s no power to flush itÂ´s caches with?
As you said, battery backup is a must for RAM disks or RAM write back cache. It's not technically required for disks (spinning or flash).
Haven't read up enough on it to know why Oracle use what they use, I guess reliability. I'd still expect the Vertex to work correctly most of the time, but if in 1 in a 1000 or 10000 power outages it doesn't store the data correctly thats enough for Oracle to choose a different more reliably technology. But also explains why you can't intentionally create a data error by turning the power off on your test rig...Sebulon said:How come everyone else, including SUN/Oracle uses battery-backing for their logs? Well, in most cases itÂ´s about battery-backed RAM, which would be useless otherwise, but if a consumer-grade SSD would do the same job, even without battery-backing, then IÂ´m having a hard time understanding why they donÂ´t just use that instead. Imagine the savings for Oracle if they ditched the ZEUSÂ´s for ordinary VertexÂ´s, and getting better performance at the same time.
I have thought that you definitely needed battery-backing to maintain a consistant ZIL. Say you build a database NAS, or a VMWare datastore, export over NFS and once youÂ´ve gone into production and that database/datastore has grown to 50TB- then you have a power-outage, which does happen to everyone from time to time. What happens then?
For everyone that wants to build mission critical systems based on FreeBSD and ZFS, is it really enough relying on a non battery-backed SSD?