[DRBD-user] difference in speed when using external metadata

J. Ryan Earl oss at jryanearl.us
Fri Jan 14 19:49:28 CET 2011

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On Thu, Jan 13, 2011 at 2:07 PM, Mark Morlino <mrmorlino at alaska.edu> wrote:

> On Mon, Jan 10, 2011 at 10:31 AM, Bart Coninckx
> <bart.coninckx at telenet.be> wrote:
> > On Monday 10 January 2011 19:40:17 J. Ryan Earl wrote:
> >> On Sun, Jan 9, 2011 at 10:03 AM, Bart Coninckx
> > <bart.coninckx at telenet.be>wrote:
> >> > Hi all,
> >> >
> >> > am reading the Novell docs on DRBD and one of the ways they mention to
> >> > speed
> >> > up DRBD is using external metadata. Possibly a hard question to answer
> >> > but could someone indicate by what degree this would enhance speed
> >> > (provided using
> >> > a sufficient fast disk for the metadata, like a SSD)?
> >> >
> >> > thx!
> >> >
> >> > Bart
> >>
> >> A RAID controller with a backed-write cache essentially nullifies
> >> any advantage there.  A RAID controller that can instantly return on
> >> FUA-writes is what you want.
> >> -JR
> >
> > Hi,
> >
> > so the essence of your answer is that this is not a matter of having
> metadata
> > internal or external, but rather of having the right RAID controller?
> > Do you have examples of these in the major brands (HP, Dell, ...) ?
> >
> > thx!
> >
> >
> >
> > B.
> > _______________________________________________
> > drbd-user mailing list
> > drbd-user at lists.linbit.com
> > http://lists.linbit.com/mailman/listinfo/drbd-user
> >
> I would be interested in this information too (in my case I'd be
> interested in any supermicro controllers that meet this requirement).
> If I understand correctly, an FUA-write is a write-through request.
> Doesn't that mean that the controller should not return until
> everything has been physically written to the disk? It seems to me
> that in order to return instantly on an FUA-write, the controller
> would have to ignore the FUA request and return once the data has been
> written to the cache. I don't see how it can both force physical disk
> access and return instantly. Perhaps I'm missing something, it
> certainly wouldn't be the first time.

So a backed-cache is a cache where power-failure won't cause the cache to
lose it's contents.  I'll go with the example of a HP P410i SmartArray
Controller with 1GB of flash-backed write cache.  In this case there is 1GB
of DDR2-533 ECC memory on the RAID controller with 4.2GB/sec of bandwidth.
 To the OS, this particular card doesn't even register as having a
write-cache, it indicates that all writes are write-through.  However, the
hardware does cache the writes to this memory.  In the event of a power
failure, there are capacitors that hold enough charge to write the contents
of the DDR2 memory to a non-volitate flash memory device.  Thus the reason
it doesn't wait for disk.  When power is restored, it'll finish writing the
contents of the flash to disk.

So, the raid controller is returning as soon as it writes the request to
write-cache, there is no latency component waiting for disk.  In my
tests, synchronous writes would return in under a microsecond versus the
milliseconds you'd wait without a write cache on a mechanical drive.  Also,
SSDs can vary, but if you want rediculously sick I/O I'd suggest the
PCI-express SSD cards like ioDrives from ioFusion.

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