SSDs on RAID

12/12/20 - storage,kb

RAID arrays have been the foundation of storage systems since the 90’s, ranging from Small businesses to Enterprise environments. Originating from Berkeley in the 80’s, the concept of a RAID allowed to create storage redundancy and striping across multiple disks to allow storage from a single pool, instead of organizing between multiple separate volumes. Often times, creating an array with multiple inexpensive disks will outperform a single, larger disk in a large number of factors.

Not always expressly mentioned, another benefit of an array is an increase of read (and sometimes, write) speeds. Couple this with a larger single volume, and redundancy to protect against physical disk failure, and a RAID is an easy decision.

With the cost of SSD media falling lower and becoming more competitive with spindle disks (HDDs), some people ask if a RAID is still relevant? Well, of course it is. The same RAID configurations used for HDDs also work with SSDs, but the necessity for a RAID10 in smaller – and even much larger – environments isn’t nearly as necessary with SSDs. The main benefits of a RAID10 were the increased redundancy and read/write speed for HDDs, but if you only require some redundancy, a RAID6 will benefit from both the natural speed of SSDs. That’s not to say that RAID10 is irrelevant for SSDs – in fact, it is still a very important option for large arrays that require the best possible redundancy available.

RAID configurations

The most common RAID configurations are the base RAID 1, 5, 6, & 10 (or 0+1, depending on the RAID card).

• RAID-0 is an option, but is essentially “all gas, no brakes”: all space and speed, but no redundancy – you will likely only see this from a consumer or end-user aspect, these days.
• RAID-1 is a mirror, whether between two drives or 5, you only use the storage space of a single drive, but can handle disk failures up to the final available disk. Performance gain is largely in the Read aspect of IOPS (with a potential of up to 1x per disk), but write speed remains the same as a single disk.
• RAID-5 requires at least 3 drives, and you lose the space equal to one drive for parity. This doesn’t mean all the parity is stored on a single drive – quite the contrary, it’s spread and striped across all the disks in the array. Simple math is to just add the total capacity of all the drives, minus 1. Regarding performance, read speed gains roughly a 1x per disk, minus 1 disk for parity (much like the storage calculations), but there is little to no write speed gain.
• RAID-6 is the same as a RAID-5, but using 1 extra drive for parity – this means there is a minimum requirement for 4 drives to construct this array. The calculation also comes in play for performance and size, as well: negate the speed and size for both parity disks when calculating. With the increased parity over a RAID-5, this leads to using SSDs for the best option (or, also, the best option for SSDs in small to medium size businesses).
• RAID-10 (1+0 & 0+1) is the fastest and most redundant of the common options, but also will cost the most. Essentially, take the total capacity of an even number of drives, then cut it in half – there is your workable storage space. Read performance gain is 1x per disk and write performance gain is 0.5x per disk. There is a difference of RAID-10 configurations, of a 1+0 and a 0+1, but this is largely involving exactly how the physical drives are mirrored and striped (or striped and mirrored, respectively), which will lead to a slight change in performance and redundancy, an exactly how failures will work. If you are forced to use HDDs, a RAID-19 will be the best option in this scenario. SSDs will also gain from this configuration, but for small to medium sized businesses, many will view it as a waste of resources.

Which is the better option?

When it comes down to it, RAID-6 is a solid option for SSDs in many configurations, but HDDs should not be written off as an option. HDDs, while vastly cheaper – and slower – they have a propensity to last longer than SSDs when there will be a large number of reads/writes. When it comes to archive/backup storage, HDDs will always be the better option. However, when you need application or database performance, SSDs in a RAID-6 will outperform a large HDD RAID-10 array any day of the week.