Understanding RAID 5 with SSD (Solid State Drives)


  • Service Provider

    The question often comes up as to why RAID 5 is so dramatically warned against and considered deprecated for use with traditional Winchester hard drives (aka spinning rust) and yet is often recommended for use with more modern SSDs (solid state drives.) Like with anything, it is actually a combination of many factors that come together to make one use case so bad and the other so generally good. Here is the run down:

    • UREs are the primary risk factor for traditional hard drives in RAID 5 arrays, but UREs are not a risk (so far) on SSDs. So this one fact alone completely changes the "risk game" between Winchester drives and SSDs
    • Time to Resilver is hugely reduced with traditional arrays often taking days or even weeks to resilver. The move to SSDs often cuts that to a small fraction of the original time. That resilver time is not just a performance impact to the environment, in some cases actually making the array useless until it has completely, but also means that the array is completely at risk of secondary drive failure during that window. Reducing that window greatly reduces that risk.
    • Resilver Impact is much reduced as SSDs handle non-sequential data access so well meaning that even during a typical resilver an all SSD RAID 5 array may continue to function extremely well while still performing a high speed resilver operation meaning that the risk of performance impact to the environment is much smaller.
    • Parity Resilver secondary drive failure risk does not exist. This is a rather sizeable risk to Winchester drives. The parity resilver operation often induces other drives to fail during the resilver operation due to the large strain placed on them for the operation. This does not impact SSDs making the resilver operation far safer.
    • Performance is very different between Winchester drives and SSDs. The move from Winchester drives to SSD is a many orders of magnitude jump in performance. The write performance difference between RAID 5 and its key competitor, RAID 10, is small by comparison. So while the latency impact of a parity calculation is great in relation to the IOPS of the SSDs, the overall speed increase is generally so immense that the performance loss to the parity system is often of no consequence except in the most demanding environments and in those environments it is common to move to different models of data protection other than traditionally managed RAID arrays.
    • Cost is very different with SSDs having a high capacity cost but a low performance cost, the opposite of Winchester drives. This means that using RAID 5 often results in a large cost savings while maintaining high performance and high reliability that does not exist with Winchester drives.


  • Wish I'd known this when I purchased a new server in January and it makes the cost difference between SSD and traditional drives much closer (since you don't need as many SSD drives for the same useable storage).

    This is a real eye opener for me.

    So is RAID 5 set to become the de facto choice for the majority of SMBs now?



  • @Carnival-Boy Only if you're using SSD's.

    If your running a classic hard drive(spinning rust) go with RAID 10.

    Maybe if the price of SSDs come down even more would RAID 10 be viable for use with SSDs



  • Yes. I'm assuming that SSDs will become the norm soon, and hence RAID 5 will become the primary choice.

    To answer your last point, what's the pros of RAID 10 for SSD?


  • Service Provider

    @Carnival-Boy said:

    To answer your last point, what's the pros of RAID 10 for SSD?

    The same as for spinning rust. The cons though (cost and size) outweigh the benefits. You get faster resilver, faster I/O, etc. what is underneath the RAID does not change those facts because that is part of how RAID works.



  • I'm guessing that RAID 6 would be more beneficial compared to the cost of RAID 10 with SSD. As Scott mentioned, if you need more performance than RAID 5 SSD can provide, you're probably looking at other options already anyway.

    I bet the power and cooling requirements for RAID 5 (or 6) are noticeably less than spinning rust at the same usable capacity.



  • Great Post.

    I've often seen people over at spiceworks blindly put down anyone suggesting RAID 5 as an option when brought up in SSD discussions. They've learned RAID 5 = bad period not RAID 5 = bad because of the above reasons.


  • Service Provider

    @Drew - Ahh, internet mob mentality.

    People want to be liked, so they will parrot whatever the current gospel wisdom is regardless of understanding.


  • Service Provider

    @Breffni-Potter said:

    @Drew - Ahh, internet mob mentality.

    So much this.


  • Service Provider

    @JaredBusch said:

    @Breffni-Potter said:

    @Drew - Ahh, internet mob mentality.

    So much this.

    Actually wait, that applies to anything, just look at most users. Not just internet mobs.


  • Service Provider

    @Breffni-Potter said:

    Actually wait, that applies to anything, just look at most users. Not just internet mobs.

    It is widely applicable, but simply more pronounced in the pseudo anonymous realm of the internet.


  • Service Provider

    @Carnival-Boy said:

    So is RAID 5 set to become the de facto choice for the majority of SMBs now?

    I would say so as long as to become is dependent on SSDs also becoming the standard, which they are set to be but are not yet. I'd say SSDs as the SMB default for new installs is likely close to two years ago, but getting close in IT terms for sure.


  • Service Provider

    @Carnival-Boy said:

    Yes. I'm assuming that SSDs will become the norm soon, and hence RAID 5 will become the primary choice.

    Yes, and like on Winchester drives, RAID 5 will probably remain the standard for many years.

    Unlike Winchester drives where UREs and massive capacity caused RAID 5 to "fail" on effectiveness, by guess is that for SSDs the disruptor is going to be a move to inclusive SSD systems like FusionIO that are not treated like traditional drives and the RAID is built into the system.


  • Service Provider

    @Drew said:

    Great Post.

    I've often seen people over at spiceworks blindly put down anyone suggesting RAID 5 as an option when brought up in SSD discussions. They've learned RAID 5 = bad period not RAID 5 = bad because of the above reasons.

    Which is how we got into the problematic position in the first place because everyone read the white papers from 1998 that said "RAID 5 is generally the best choice because of cost savings" and interpreted that as RAID 5 = good and failed to learn the reasons that RAID 5 was chosen on small Winchester disks in the late 1990s.

    It's amazing how quickly the pattern of attempting to learn IT as a simple set of rules rather than "good starting points combined with an understanding of the technology so that it is understood when to deviate from the norm."

    The worst part is probably that so many people think that IT is so simplistic that we can have a simple set of rules that anyone without training could follow blindly rather than needing to actually understand the technology and apply that knowledge as needed. This is what leads, I think, to say many posts asking "What's the best X" as if there is always one single answer and that the secret to IT is getting this closely guarded knowledge rather than needing to understand products, techniques, risk, cost, etc.


  • cid:7:privileges:read

    @Breffni-Potter I agree with everything Scott says, with the exception of stopping at strange bars in the middle of the jungle at 10:00 at night.


  • Service Provider

    @Bob-Beatty said:

    @Breffni-Potter I agree with everything Scott says, with the exception of stopping at strange bars in the middle of the jungle at 10:00 at night.

    That was more like "on the edge of the jungle" and @pchiodo will back me up that that was a great decision.


  • Service Provider

    @Bob-Beatty said:

    @Breffni-Potter I agree with everything Scott says, with the exception of stopping at strange bars in the middle of the jungle at 10:00 at night.

    Are you honestly telling me it was just the "one" thing you disagreed with? 🙂

    I suspect there might be a few more bar visits you would object to if you cast your mind back.


  • cid:7:privileges:read

    @Breffni-Potter said:

    @Bob-Beatty said:

    @Breffni-Potter I agree with everything Scott says, with the exception of stopping at strange bars in the middle of the jungle at 10:00 at night.

    Are you honestly telling me it was just the "one" thing you disagreed with? 🙂

    I suspect there might be a few more bar visits you would object to if you cast your mind back.

    haha! I.. cant.... remember too many bar visits with Scott, or I may agree with your comment... 😛



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