> From: Richard Elling [mailto:richard.ell...@gmail.com] > > On Apr 17, 2010, at 11:51 AM, Edward Ned Harvey wrote: > > > For zpool < 19, which includes all present releases of Solaris 10 and > > Opensolaris 2009.06, it is critical to mirror your ZIL log device. A > failed > > unmirrored log device would be the permanent death of the pool. > > I do not believe this is a true statement. In large part it will depend > on > the nature of the failure -- all failures are not created equal. It has > also > been shown that such pools are recoverable, albeit with tedious, manual > procedures required. Rather than saying this is a "critical" issue, I > could > say it is "preferred." Indeed, there are *many* SPOFs in the typical > system > (any x86 system) which can be considered similarly "critical."
Could you please describe a type of failure of an unmirrored log device in zpool < 19 which does not result in the pool being faulted and unable to import? I don't know of any. If you have a faulted zpool < 19, due to a faulted nonmirrored log device, could you describe how it's possible to recover that pool? I know I tried and couldn't do it, but then again, it was only a test pool. I only dedicated an hour of labor to trying. > Finally, you have choices -- you can use an HBA with nonvolatile write > cache and avoid the need for separate log device. The HBA with nonvolatile cache gains a lot over just plain disks. By my measurement, 2x-3x faster for sync writes, but no improvement for async writes, or any reads. But it's not as effective as using a dedicated SSD for log device. By my measurement, using a SSD for log device (with all the HBA write cache disabled) was about 3x-4x faster than just plain disks for sync writes, but no different for async writes, or any reads. I agree with you, HBA nonvolatile write cache is an option. It's cheaper than buying an SSD, and it doesn't consume a slot. Better than nothing. Depends on what your design requirements are, and how much you care about the sync write performance. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
