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

Rob Kampen rkampen at kampensonline.com
Thu Dec 10 18:49:15 UTC 2009


Chan Chung Hang Christopher wrote:
> 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.
>   
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??
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>   
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