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From Flavio Junqueira <...@yahoo-inc.com>
Subject Re: zookeeper on SSD
Date Wed, 17 Oct 2012 20:42:20 GMT
Let me add one observation to this thread. If you check slide 69 of this presentation:


the graph shows that not writing to disk (net only) does not actually improve write latency
much, unless your disk write buffer is turned off. Unless there has been some important performance
improvement I missed, it doesn't look like a faster device for the transaction log would be
able to improve latency much at this point. Does it sound right?


On Oct 4, 2012, at 6:05 PM, Ben Bangert wrote:

> On Oct 3, 2012, at 6:13 PM, Patrick Hunt <phunt@apache.org> wrote:
>> My experience with SSDs and ZK has been discouraging. SSDs have some
>> really terrible corner cases for latency. I've seen them take 40+
>> seconds (that's not a mistake - seconds) for fsync to complete. When
>> this happened (every few hours) all of the sessions would timeout.
>> See this article:
>> http://storagemojo.com/2012/06/07/the-ssd-write-cliff-in-real-life/
> It's worth noting that these tests are all on Enterprise SSD products, which have actually
been lagging some of the advances the SSD controller folks have been making. I've had the
same corner case on my own desktop SSD in the past with a huge write cliff, but this has gone
away with some of the later heavily over-provisioned SSD's I've bought, such as this OWC 6G
one I'm using.
> Course, these Enterprise folks are the same that prefer to scale vertically than horizontally
using cheaper commodity hardware. The most useful factors to look at when choosing the SSD
are the write amplification factor (http://www.anandtech.com/show/5719/ocz-vertex-4-review-256gb-512gb),
and how it handles the case when the drive runs out of free space (and thus has to garbage
collect resulting in the write cliff). An over-provisioned drive can avoid the write-cliff
because a chunk of the drive is reserved in advance to prevent it from ever getting completely
full. See results here:
> Over-provisioned SSD:
> http://macperformanceguide.com/SSD-RealWorld-BeforeAfter-OWC.html
> Non-overprovisioned SSD:
> http://macperformanceguide.com/SSD-RealWorld-BeforeAfter-CrucialRealSSD.html
> If you look through, there's some very worrying write-cliffs that are very apparent in
SSD's that aren't over-provisioned, and they easily fail to perform as well as a RAID of platter
> The other thing about the storagemojo article worth thinking about is whether you're
actually going to buy a 12+ disk array for a faster ZK log... or are actually comparing a
single platter disk vs. a single SSD.
> Cheers,
> Ben

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