Re: clustered MD - beyond RAID1

[NeilBrown <neilb@suse.de>]

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On Tue, Dec 22 2015, Tejas Rao wrote:

> GPFS guarantees that only one node will write to a linux block device 
> using disk leases.

Do you have a reference to documentation explaining that?
A few moments searching the internet suggests that a "disk lease" is
much like a heart-beat.  A node uses it to say "I'm still alive, please
don't ignore me".  I could find no evidence that only one node could
hold a disk lease at any time.

NeilBrown


>                    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
>
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