[Drbd-dev] [PATCH] expand section on throughput tuning to highlight prime usecase of external metadata, florians comments applied, added section replacing metadata
Pawel Jakub Dawidek
pjd at FreeBSD.org
Tue Aug 16 17:25:06 CEST 2011
On Tue, Aug 16, 2011 at 03:08:33PM +0200, Lars Ellenberg wrote:
> I suggest to point out that this is for _linear streaming writes_,
> which, once they leave the covered area, need one activity log
> transaction every 4 MiB.
> Other, more localized, usage patterns need less AL-transactions per
> amount of changes, and are less impacted, or virtually not at all,
> depending on the change rate and size of the working set, and the
> size of the activity log.
> The impact heavily depends on the overall AL-transaction latency, which
> is why write cache enabled controllers do much better. Even better than
> a dedicated disk, usually, which may no longer suffer from seek time,
> but still has access and processing time up to two orders of magnitude
> higher than a decent write cache.
BTW. How do you ensure AL update "reached the platters"? Do you flush
disk write cache after every AL update?
> > +see writes of 70 MB/s with DRBD. This is purely caused by head seeks; data
> > +writes have to be followed by activity log updates and data writes can only
> > +continue after the meta data update has _reached the platters_. With RAID-0,
Pawel Jakub Dawidek http://www.wheelsystems.com
FreeBSD committer http://www.FreeBSD.org
Am I Evil? Yes, I Am! http://yomoli.com
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