[linux-lvm] Will LVM2 ever be able to do striped mirrors "raid 10"?

[Bryn M. Reeves]

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John Hughes wrote:
> malahal at us.ibm.com wrote:
>> John Hughes [john at Calva.COM] wrote:
>>  
>>> John Hughes wrote:
>>>    
>>>>                if (lp->stripes > 1) {
>>>>                        log_error("mirrors and stripes are currently "
>>>>                                  "incompatible");
>>>>                        return 0;
>>>>                }
>>>>
>>>> Should I just stick with mdadm for my mirroring and striping needs?
>>>>       
>>
>> You can do it today with some hassle or wait until someone implements a
>> feature called 'generic layering'. The feature really means, treat some
>> LVs as PVs!
>>
>> How can you do raid10 today? Create two raid0 LVs. Lets us call these
>> lvgroup0/lv0 and lvgroup0/lv1. Now create raid1 lv in lvgroup1 where
>> lvgroup1's PVs are lvgroup0/lv0 and lvgroup0/lv1.
>>
>> Isn't that a rai10 volume?

No, it's a RAID 0+1 (RAID01/mirror of stripes).

> To increase the chances of surviving a double-disk failure it would be
> better to raid-0 a bunch of raid-1's.

This is RAID1+0 (RAID10/stripe of mirrors) - it's usually preferable to
0+1 not only because of the improved redundancy but also the individual
mirror sets can have failures and recover independently. When a strip
set in a RAID0+1 fails the entire mirror must be re-synchronised giving
longer recovery times and more performance degradation during the rebuild.

> Are we sure there are no deadlock problems with LVM2 layered on top of
> LVM2?

Stacked volumes work fine, there's just limited support in the tools for
creating them without going to a lot of manual effort at the moment.

Regards,
Bryn.
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