Failing ZFS log devices/panic

[Grant Gray]

----- On 31 Aug, 2018, at 1:04 PM, M. Casper Lewis mclewis at genomecenter.ucdavis.edu wrote:

> On Thu, Aug 30, 2018 at 07:58:05PM -0700, M. Casper Lewis wrote:
>> On Fri, Aug 31, 2018 at 11:35:12AM +1000, Grant Gray wrote:
>> > I've currently got an issue between some HGST SATA disks and a SAS3008 HBA
>> > where mixing SAS and SATA disks on the same port results in intermittent
>> > I/O errors on the HGST SATA disks, but not other SAS devices.
>> 
>> This sounds like what we are seeing, but we're not mixing SAS and SATA.
> 
> I forgot to add:  Why are the log devices showing problems and not the
> cache devices?  Same drives, same bus.  Different access pattern, sure.
> 
> --
> M. Casper Lewis                     |   mclewis at ucdavis.edu
> Systems Administrator               |   Voice: (530) 754-7978
> Genome Center                       |
> University of California, Davis     |

It could be a function of synchronous write load. Log devices must be coherent. Cache devices can back-fill (I don't know how the ZFS algorithm works, but I imagine pages will continue to hit the L1 ARC before and during demotion to the L2 ARC).