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.