On Mon, Oct 06, 2008 at 05:38:33PM -0400, Brian Hechinger wrote:
> On Sun, Oct 05, 2008 at 11:30:54PM -0500, Nicolas Williams wrote:
> > There have been threads about adding a feature to support slow mirror
> > devices that don't stay synced synchronously.  At least IIRC.  That
> > would help.  But then, if the pool is busy writing then your slow ZIL
> 
> That would definitely be a great help.
> 
> > mirrors would generally be out of sync, thus being of no help in the
> > even of a power failure given fast slog devices that don't survive power
> > failure.
> 
> Maybe not, but it would at least save *something* as opposed to not saving
> anything at all.  Still, with enough UPS power, there should be at least
> enough run time left to get the rest of the ZIL to the disk mirror.

Yes.  But again, you get somewhat more protection from writing to a
write-biased SSD in that once the ZIL bits are committed then you get
protection from panics in the OS too, not just power failure.

> > Also, using remote devices for a ZIL may defeat the purpose of fast
> > ZILs, even if the actual devices are fast, because what really matters
> > here is latency, and the farther the device, the higher the latency.
> 
> 4Gb FC is slow and low latency?  Tell that to all my local fast disks that
> are attached via FC. :)

The comparison was to RAM, not "local fast disks."

I'm pretty sure that local RAM beats remote-anything, no matter what the
"anything" (as long as it isn't RAM) and what the protocol to get to it
(as long as it isn't a normal backplane).  (You could claim with NUMA
memory can be remote, so let's say that for a reasonable value of
"remote.")

> > Yes, it's pretty smart.  Add UPS and it's sortof like battery-backed
> > RAM.  You can probably get a good enough reliability rate out of this
> > for your purposes, though actual slog devices would be better if you can
> > afford them.
> 
> Or would they?  A box dedicated to being a RAM based slog is going to be
> faster than any SSD would be.  Especially if you make the expensive jump
> to 8Gb FC.

Unless the SSD had a battery-backed RAM cache, or were based entierly on
battery-backed RAM (but then you have to worry about battery upkeep).

To me this is a performance/reliability trade-off.  RAM slogs mirrored
in cluster + UPS -> very fast, works as well as the UPS.  Write-biased
flash slogs -> fast, no UPS to worry about.

Nico
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