[DRBD-user] external metadata on ssd vs bcache

Lars Ellenberg lars.ellenberg at linbit.com
Thu Feb 28 15:25:16 CET 2013

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, Feb 28, 2013 at 09:49:16AM +0100, Lionel Sausin wrote:
> Dear Arnold,
> Thanks for your feedback.
> It's interesting because, normally, writes do not directly translate
> to head seeks (thanks to dirty pages, caches, NCQ, firmware-level
> optimization...), and ideally barriers should be disabled (and
> caches reliable).
> Florian Haas once suggested[1] that "if using external metadata
> actually improves your performance versus internal metadata, you
> have underlying performance problems to fix."

Well, yes and no ;-)

If you have a RAID 6 backend and don't want to invest in decent
controller cache, using an external RAID 1 or 10 volume for meta data
can significantly improve your performance, in case you should be
limited by the activity log (your work set is much larger than what
the activity log typically covers).

Such usage patterns are typically, by design of the activity log, random
writes. If you have lot of random writes however, your performance will
improve even more, if you invest in that decent controller cache, and
once you have it, the single write transaction latency is so much
reduced that the benefit of the external meta data is negligible again.

Large streaming writes may also be throttled by activity log updates,
but the same applies: even a moderately sized controller cache will help.

And then there are the recent activity log improvements with DRBD 8.4.3,
which boost you performance for any scenario where your working set
often leaves the activity log, as long as you can keep a bit of IO
in-flight (not a single synchronous writer):


Note that the given performance numbers
are for the *worst case* usage pattern:
small, 100% random writes covering the full range block numbers.

Though the improvement vs older drbd versions is not only for random
writes, but for any writes causing activity log updates.

: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

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