Re: clustered MD - beyond RAID1

[Tejas Rao <raot@bnl.gov>]

GPFS guarantees that only one node will write to a linux block device 
using disk leases. Only a node with a disk lease has the right to submit 
I/O and disk leases expire every 30 secs and needs to be renewed. Lustre 
and other distributed file systems have other ways of handing this.

Using md devices in a shared/clustered environment is something not 
supported by Redhat on RHEL6 or RHEL7 kernels, so this is something we 
would not try in our production environments.

Tejas.

On 12/21/2015 15:47, NeilBrown wrote:
> On Tue, Dec 22 2015, Tejas Rao wrote:
>
>> What if the application is doing the locking and making sure that only 1
>> node writes to a md device at a time? Will this work? How are rebuilds
>> handled? This would be helpful with distributed filesystems like
>> GPFS/lustre etc.
>>
> You would also need to make sure that the filesystem only wrote from a
> single node at a time (or access the block device directly).  I doubt
> GPFS/lustre make any promise like that, but I'm happy to be educated.
>
> rebuilds are handled by using a cluster-wide lock to block all writes to
> a range of addresses while those stripes are repaired.
>
> NeilBrown