Re: clustered MD - beyond RAID1

[Aaron Knister <aaron.s.knister@nasa.gov>]

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Hi Tejas et al,

I'm fairly confident in saying that GPFS can have many servers actively 
writing to a given NSD (LUN) at any given time. In our production 
environment the NSDs have 6 servers defined and clients more or less 
write to whichever one their little hearts desire. Do you think it's 
possible that the explicit primary/secondary concept is from an older 
version of GPFS? I'm not sure what the locking granularity is for 
NSDs/disks, but even if it's a single GPFS FS block and that block size 
corresponds to the stripe width of the array I'm pretty nervous relying 
on that assumption for data integrity :)

The use case here is creating effectively highly available block storage 
from shared JBODs for use by VMs on the servers as well as to be 
exported to other nodes. The filesystem we're using for this is actually 
GPFS. The intent was to use RAID6 in an active/active fashion on two 
nodes sharing a common set of disks. The active/active was in an effort 
to simplify the configuration.

I'm curious now, Redhat doesn't support SW raid failover? I did some 
googling and found this:

https://access.redhat.com/solutions/231643

While I can't read the solution I have to figure that they're now 
supporting that. I might actually explore that for this project.

-Aaron

On 12/21/15 8:09 PM, Tejas Rao wrote:
> Each GPFS disk (block device) has a list of servers associated with it.
> When the first storage server fails (expired disk lease), the storage
> node is expelled and a different server which also sees the shared
> storage will do I/O.
>
> There is a "leaseRecoveryWait" parameter which tells the filesystem
> manager to wait for few seconds to allow the expelled node to complete
> any I/O in flight to the shared storage device to avoid any out of order
> i/O. After this wait time, the filesystem manager completes recovery on
> the failed node, replaying journal logs, freeing up shared tokens/locks
> etc. After the recovery is complete a different storage node will do
> I/O. There is a concept of primary/secondary servers for a given block
> device. The secondary server will only do I/O when the primary server
> has failed and this has been confirmed.
>
> See "servers=ServerList" in man page for mmcrnsd. ( I don't think I am
> allowed to send web links)
>
> We currently have 10's of petabytes in production using linux md raid.
> We are currently not sharing md devices, only hardware raid block
> devices are shared. In our experience hardware raid controllers are
> expensive. Linux raid has worked well over the years and performance is
> very good as GPFS coalesces I/O in large filesystem blocksize blocks
> (8MB) and if aligned properly eliminate RMW (doing full stripe writes)
> and the need for NVRAM (unless someone is doing POSIX fsync).
>
> In the future ,we would prefer to use linux raid (RAID6) in a shared
> environment shielding us against server failures. Unfortunately we can
> only do this after Redhat supports such an environment with linux raid.
> Currently they do not support this even in an active/passive environment
> (only one server can have a md device assembled and active regardless).
>
> Tejas.
>
> On 12/21/2015 17:03, NeilBrown wrote:
>  > 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
>

-- 
Aaron Knister
NASA Center for Climate Simulation (Code 606.2)
Goddard Space Flight Center
(301) 286-2776


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