Mathieu Baudier wrote: >> LVM like md raid and drbd is a layered block device and >> If you turn the wire caches off on the HDs then there is no problem, >> but HDs aren't designed to perform to spec with the write cache >> disabled they expect important data is written with FUA access (forced >> unit access), so performance will be terrible. >> > > I hope that I'm not going too much off topic here, but I'm getting > worried not to be sure to understand, especially when it has to do > with data safety: > > Considering a stack of: > - ext3 > - on top of LVM2 > - on top of software RAID1 > - on top of regular SATA disks (no hardware RAID) > is it "safe" to have the HD cache enabled? > > (Note: ext3, not XFS, hence the possible off-topic...) > Nothing is safe once device-mapper is involved. > In other words, is this discussion about barriers, etc. only relevant to XFS? No, it applies to all filesystems. Prior to barriers, fsync/fsyncdata lies. See the man page for fsync.