What is the difference between SAN, NAS and DAS?
The difference between SAN, NAS and DAS
direct-attached-storagenetwork-attached-storagestoragestorage-area-network
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I think you're going to have to ask Softlayer directly, or try them out ,but you can rule out iSCSI SAN, not because of performance, but because you're looking at a shared filesystem to maximise utilisation, rather than raw block access for performance.
NAS will be easy to use, mount a shared filesystem on each boxes, and just copy the files to that filesystem.
Evault I've not got a clue about but it looks like it's Softlayers brand for snapshot backups of servers, so it does a full copy of the server without downtime. It sounds like it does de-duplication of data, which could generate significant space savings (and so cost-savings) for you.
Cloud storage is generally a term for storage accessed using web protocols like webdav rather than NFS or CIFS for NAS storage, you'll possibly have to run dedicated scripts to copy files to and from it depending how Softlayer have it running.
In terms of performance, each one of these options could run just as fast as any of the others depending how Softlayer actually have it configured and the hardware behind it, but the Evault system sounds like it's their own preferred option for your requirements.
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 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 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 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; MySQL, SimpleDB, 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.
Best Answer
First it is best to define the difference between a block device and filesystem. This is easier grasped if you are familiar with UNIX because it makes an objective distinction between the two things. Still the same applies to Windows.
/dev/sda
for a disk or/dev/sda1
for a partition on that disk.mount /dev/sda1 /mnt/somepath
.With those terms in mind it's then easier to see the distinction between the following.