[OpenIndiana-discuss] What happens when a ZIL drive dies?

[Dan Swartzendruber]

On 6/4/2012 11:56 AM, Richard Elling wrote:
> On Jun 4, 2012, at 8:24 AM, Nick Hall wrote:
> For NFS workloads, the ZIL implements the synchronous semantics between
> the NFS server and client. The best way to get better performance is to have the
> client run in async mode when possible (Solaris clients do this automatically, and
> have for a very long time, Linux... not so much).
>
> The risk is that the server unexpectedly reboots and the synchronous writes from
> the client are lost. In that case, the client thinks data is written, but it is not. The
> server is happy either way... it is the client that is sad.
>    
The most annoying client in this respect is ESXi, which insists on doing 
sync operations.  I understand the logic there - unlike, say, an 
application which can decide to do async operations, ESXi is using NFS 
as the backing store for virtual disks, so when a client (windows, 
linux, whatever) does disk writes to virtualized SCSI controller (frex), 
the guest may be doing writes on behalf of a journalized filesystem 
which is doing writes in a specific order, possibly even with write 
barriers.  In that case, cheating and forcing the writes to be 
asynchronous (say by 'sync=disabled') can in fact cause guest filesystem 
corruption.  I can't afford a high quality SSD to reduce latency, so I 
made an informed decision to disable sync mode.  The mitigation for me 
is that I do zfs snapshots every night of the ESXi datastore,  so the 
worst case is losing a day's work.  Given this is for a home/soho setup, 
and given that the openindiana SAN is on a hefty UPS, I'm willing to 
take the chance.