What Makes Cloud Storage (Amazon AWS, Microsoft Azure, google Apps) different from Traditional Data center storage networking (SAN and NAS)?
Some confusion because of my question
so to make it simple :
"What kind
of storage do big cloud providers use and why?"
As far as i understand, however I am
not able to find any kind of official Storage networking differences between typical
data centers and clouds, all cloud providers are using DAS different from the typical
data centers.
Even DAS has many disadvantages
than SAN or NAS, i want to learn the details why clouds using DAS either for storage or
application purposes.
Any resource or
description will be appreciated to make me
clear.
EDIT: While
reading the paper "Networking Challenges and Resultant Approaches for Large Scale Cloud
Construction,David Bernstein and Erik Ludvigson (Cisco)" they mention
that,
Curiously we do not see Clouds from the major providers using NAS or SAN. The
typical Cloud architecture uses DAS, which is not typical of Datacenter storages
approaches.
But
here there is a conflict: in my opinion and also stated later in the paper, Clouds
should use SAN or NAS because of DAS is not appropriate when a VM moves to another
server yet still needs to access storage from original
server.
What are other reasons effects clouds to
prefer DAS, NAS or SAN?
what kind of storage do big cloud providers use and
why?
Answer
This answer has been edited
after the question was
clarified.
What are other reasons effects clouds to prefer
DAS
Where "DAS"
means Direct Attached Storage, i.e. SATA or SAS harddisk
drives.
Cloud vendors all use DAS because it
offers order-of-magnitude improvements in price/performance. It is a case of
rel="nofollow noreferrer">scaling
horizontally.
In short, SATA harddisk
drives and SATA controllers are cheap commodities. They are mass-market products, and
are priced very low. By building a large cluster of cheap PCs with cheap SATA drives,
Google, Amazon and others obtain vast capacity at a very low price point. They then add
their own software layer on top. Their software does multi-server replication for
performance and reliability, monitoring, re-balancing replication after hardware
failure, and other things.
You could
take a look at MogileFS as a simpler representative of the kind of software
that Google, Amazon and others use for storage. It's a different implementation of
course, but it shares many of the same design goals and solutions as the large-scale
systems. If you want to, here is a jumping point for learning more about
noreferrer">GoogleFS.
stated later in the paper, Clouds should use SAN or NAS because of DAS is not
appropriate when a VM moves to another
server
There are 2
reasons why SAN's are not used.
1)
Price.
SAN's are hugely expensive at large scale. While they
may be the technically "best" solution, they are typically not used at very large scale
installations due to the
cost.
2) The CAP
Theorem
noreferrer">Eric Brewer's CAP theorem shows that at very large scale you
cannot maintain strong consistency while keeping acceptable reliability, fault
tolerance, and performance. SAN's are an attempt at making strong consistency in
hardware. That may work nicely for a 5.000 server installation, but it has never been
proved to work for Google's 250.000+
servers.
Result:
So
far the cloud computing vendors have chosen to push the complexity of maintaining server
state to the application developer. Current cloud offerings do not provide consistent
state for each virtual machine. Application servers (virtual machines) may crash and
their local data be lost at any time.
Each
vendor then has their own implementation of persistent storage, which you're supposed to
use for important data. Amazon's offerings are nice examples;
and Simple Storage
Service. These offerings themselves reflect the CAP theorem -- the MySQL
instance has strong consistency, but limited scalability. SimpleDB and S3 scale
fantastically, but are only eventually consistent.
Comments
Post a Comment