Question about LVM and RAID

Robin Laing Robin.Laing at drdc-rddc.gc.ca
Thu Nov 29 20:12:50 UTC 2007


rainer wrote:

> 
> Kanwar Ranbir Sandhu wrote:
>> Hi All,
>>
>> I have an external drive cage which has been configured with two
>> separate RAID 5 arrays.  I then used LVM to create two PVs, and then
>> added the volumes together under one VG.  The whole shebang is mounted
>> on one file system (/srv).
>>
>> What would happen if one of the RAID arrays failed (e.g. two drives die
>> in RAID 5 array 1)?  Would the data be safe, would I lose all data, or
>> would I just lose the data that was on the failed array?
>>
>> I believe I would only lose the data on the failed array, but a friend
>> believe I would lose the whole lot.
>>
>> Thanks in advance!
>>
>> Regards,
>>
>> Ranbir
>>   
> 
 > Hi,
 >
 > I suppose you have only made a JBOD with LVM - no further RAID0 or such.
 >
 > With JBOD, at least the data on the left RAID5-set should be safe. The
 > data on the failed array and the bits and pieces which were on both
 > arrays would be lost. Fragmentation and the size of data could be an
 > issue - e.g. a 200 GB file on 2 x 160GB arrays would mean more than half
 > is lost... ;(
 >
 > I think your friend meant you had made a RAID0 above the RAID5-sets,
 > which would indeed mean the whole thing would be lost.
 >
 > Regards,
 > rainer
 >

To add to this.

A RAID 5 array is dependent on the number of drives where the data is 
spread across.  If there are 3 drives, then two failures is more than 
enough.  If you have 6 drives, then two drives may be okay.

It would be more useful to explain the RAID setup with the hardware, 
number of drives per RAID and how they are configured.


-- 
Robin Laing




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