[CentOS] XFS and LVM2 (possibly in the scenario of snapshots)

Chan Chung Hang Christopher christopher.chan at bradbury.edu.hk
Thu Dec 10 16:10:59 UTC 2009


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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