Isilon as well as SONAS offer node by node expansion of existing clusters. My experience from SONAS is that it takes quite a while to go through all administration steps that are required including the manual initialization of the data re-distribution. In Isilon things are little easier and adding a node is a some minutes working task.
Within SONAS storage nodes, you can currently choose between 600 GB SAS (15k rpm) and 900 GB SAS (10k rpm) drives as well as 2 or 3 TB NL-SAS drives.
- Everything you can order is a fully populated enclosure with 60 disks.
- No smaller increments. Imagine you chose 3 TB NL-SAS disk. In that case you can only grow in 180 TB increments (raw capacity).
- Only RAID6
- No SSD
Isilon offers some more options with its different node types (S-Series, X-Series, NL-Series). There are 1U, 2U and 4U enclosures available were the 2U contains 12 and the 4U enclosures have 36 disks. Only exception here is the S200 which can take 24 disks in 2U.Currently Isilon supports 500GB, 1TB, 2 TB and 3TB SATA drives as well as 300 and 600GB SAS drives. More importantly, with Isilon, you have the choice of different HDD to SSD ratios that you can choose from. Choosing SSD allows you to place IO intensive meta data on SSDs which will accelerated access in environments which large amount of small files. Of course you are also able to put performance intensive non-meta data on SSDs.
If you just want to add processing power without additional storage, that’s possible with both systems. SONAS offers the IO-Nodes that can be configured with different memory and NIC configurations. For the same purpose you would add an Acceleration Node in Isilon. Also, both systems offer the option to add an expansion node to each storage node. That is another storage enclosure, attached to a 'parent' node via redundant SAS links. This expansion drawer has no CPU or memory and is thus only extending storage capacity. The difference again is that in SONAS you have 60 disks per enclosure while Isilon has 12, 24 or 36 disks per drawer.
Because of the dual-storage-controller strategy in SONAS, you cannot just ad nodes as like in Islion. In SONAS two storage nodes and up to two storage controllers (and optionally two expansion enclosures) build on storage POD: Pool of Disks (see the following picture).
|Fig1: SONAS HW-Architecture relies on Storage PODs|
When a storage POD is full (that is, 2 storage nodes, 2 controllers, 2 expansions) you start with a new POD, containing at least a storage node and a storage controller. More likely, you would add two storage nodes and two controllers (for availability reasons). So you would typically grow in adding new whole PODs instead of single nodes. So in practice the large expansion steps in SONAS do even double.
In Isilon you can just ad node, node, node, node.... because there is more typical dual-controller dependency. If you decide to use expansion nodes, you may decide to use them for all nodes for the same disk technology or pool for not architecting an unbalanced system.