Thinking outside the square … or “how to build a small server”

June 20, 2014

Everyone today likes SSDs, but no-one wants to buy more than they need and more than they can afford. The following article shows you some options in how to build a server, small or large, using the benefits of SSD without spending a fortune. Note that the Hybrid RAID option talked about here is available on all of Adaptec’s RAID controllers from the 6405E upwards.

The underlying basis for this article is Hybrid RAID. This is unique to Adaptec controllers and simply means then when the controller finds a mirror or RAID 10 made of both SSD and HDD, it will write data to both drives (as in the traditional mirror method), but will direct all reads to the SSD, ignoring the HDD for reads. Since the one major benefit that SSD brings to the table is read speed, it makes sense to read from the fast devices, rather than trying to balance read between the devices.

This is automatic – the controller will just do this by itself when it finds the configs below, so the user does not even have to think about anything unusual in the configuration apart from the physical devices used.

So why this article? I found this doc that I wrote ages ago, and only just worked out how to put images in this blog :-)

Option 1 – Large Capacity Servers

How to make maximum use of space in a large capacity server, while still gaining a fast-booting server (to minimize downtime).
Scenario: 8-bay server. Customer wants to gain as much space as possible while still retaining fast boot time. Customer is conservative and wishes to keep OS away from data (OS in mirror and data in RAID 5 for maximum capacity) – or more likely they are stuck with the limitations of an OS that can’t handle a boot drive of more than 2TB :-)

Traditional method

The problem with this method is that is wastes two bays in the server to the OS boot drives. Therefore capacity is limited to 6 drives in R5 (N-1 means 5 drives data capacity). The server does not boot particularly quickly as it is booting from standard HDD in mirror (not a very fast form of RAID).

Hybrid RAID method

The benefits of this method are:
1. Capacity is 7 drives x usable capacity (N-1 for RAID 5 parity)
2. In this scenario I have used a 100gb SSD, but in reality a 32gb would be sufficient to load an OS, and would result in greater overall capacity in the server
3. The server will boot quickly as it is reading from an SSD during the boot process, and booting an OS mostly consists of reading from the boot disk (hence the fast SSD)
4. The unused disk space on each disk can be utilized in another array if required – some users may wish to do this, while others will opt for simplicity in design and just not utilize the unused disk space on each 3tb drive.
5. In the above scenario, a 500gb RAID5 disk could be created across the 6 unused disk segments.

Overall benefits of going from traditional to hybrid method:
1. Greater overall capacity in server
2. Faster boot time

Usage scenarios – data center or anywhere that overall capacity is of paramount importance.

Option 2 – Workstation

The workstation user of today wants a fast booting, fast application-loading system without the hassle of having to rebuild a server if a drive fails.

The advantages of this system are:
1. All reads are directed from the SSD. Therefore read speed of the workstation is dramatically improved over that of a standard single hard drive (or even a mirror consisting of two x hdd).
2. Write speed is the same as a standard workstation, but due to the cache on the controller card will be faster than a standard hdd connected to an on-board motherboard disk controller.
3. If either drive fails, the user does not lose data – they simply replace the failed drive and the array will rebuild onto the replacement drive, putting the system back the way it was before the drive failure.
4. Regarding the 200gb capacity unused on the 500gb hdd – it is not recommended to use this additional space (even though it is possible to do so) due to the fact that this data is not redundant, and failure of the hdd would result in data loss.

Option 3 – Small business server

Small business does not mean slow business. Many small business servers require at least some high-performance storage component to handle accounting software, industry-specific small database and even mail servers.
Along with that performance there is always the requirement for capacity. Even small business can easily generate several TB of data in the form of documents, photos, video etc.

The advantages of this system are:
1. The 160gb disk is big enough to create a boot volume for Windows (etc), and leave enough space for a 100gb volume for database function.
2. Because the 160gb hybrid mirror reads directly from the SSD, both OS boot time and database function will be greatly improved over a standard mirror of 2 x hdd.
3. 160gb is lost from the capacity of the data mirror, however with 3tb drives that is a small percentage of capacity loss and does not significantly impact on the usable capacity of the server.

Option 3 – Small business server – extension of the theme

Since the previous configuration requires at least a 4-port RAID card (eg 6405E), the 4th port is unused in the above configuration. It is a simple matter to extend the server to utilize all 4 ports. This would require purchasing a second 160gb SSD to mirror to the unused space from the previous example.

It might seem far more sensible to mirror the two SSD drives, then mirror the 2 x hdd drives in what would seem a far more conventional server. However, there is good reason to do the above:
1. The write speed of lower-spec solid state drives is not greatly faster than that of HDD, so there is nothing really to be gained in write speed (minor) by mirroring the two SSD.
2. Since the read speed of the SSD is so quick, putting them in a mirror will not result in much improvement over a single SSD since most reads would be concluded before the logic of the RAID card has a chance to bring the read characteristics of the second drive into play.
3. Since the focus is on read speed, having two hybrid raid arrays now gives 320gb of SSD read speed to the server, while still protecting the data on each SSD in the form of a mirror.
4. The capacity of the server (HDD space) is not sacrificed from the previous example.

Option 3 – Small business server – extension of the theme – high-speed, small capacity

This configuration focuses on performance, not capacity.
By purchasing 2 600gb SSD the user can get 1.2tb capacity from these drives. Instead of the traditional method of mirroring the drives and wasting the capacity of the second SSD, by mirroring the SSD to the HDD the user gets the full capacity of the purchased SSD.

This server is for the user who is focused on speed, not capacity, and wishes to utilize all of the SSD capacity.
Note that it would be possible in the above config to add a third SSD to the config (using the 4th port on the RAID card), to give 3 x Hybrid mirrors on a total of 3 SSD and 1 HDD.


As you can see there are plenty of configuration options you can do when you understand Hybrid RAID. While not every one of these may be useful for you, it’s good to keep in mind that there are options out there, and that you can offer the customer differentiation that may just be what he’s looking for.



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