Chan Chung Hang Christopher wrote:
No mention of barriers in the man page, I'm also getting confused. is device mapper used for software raid - i.e. /dev/mdX? If so what are the implications of barriers and where are they turned on / off? Forgive me for potential off topic, but I too run xfs on lvm which uses mapper.......risky??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.
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