Ceph hardware sizing calculator

Modification of single block will still require reading whole stripe from the disk and saving it again, or recalculating the parity block (depending on RAID implementation) and saving it to disk. Because SSDs have less problems performing random accesses it won't suffer as much performance degradation as similar array with HDDs but it won't be negligible.

The other problem you may run into is the performance of the controller itself. Cards with older chipsets can't process more than 400-500MiB/s with RAID. You definitely want to check for that.

In the end, only you can answer this question by benchmarking. Check how fast either configuration is running and use it.

I'd advise against RAID5/6, especially when it's a performance critical array.

Let's try to reply one question at a time:

• Is TRIM support necessary for modern (2015-2016 era) SSDs?

Short answer: in most cases, no. Long answer: if you reserve sufficient spare space (~20%), even consumer-grade drive usually have quite good performance consistency values (but you need to avoid the drives which, instead, choke on sustained writes). Enterprise-grade drives are even better, both because they have higher spare space by default and because their controller/firmware combo is optimized toward continuous use of the drive. For example, take a look at the S3700 drive you referenced: even without trimming, it has very good write consistency.

• Often drives are advertised as having improved built-in garbage-collection, does that obviate the need for TRIM? How does their GC process work in RAID environments

The drive garbage collector does its magic inside the drive sandbox - it does not know anything about the outside environment. This means that it is (mostly) unaffected by the RAID level of the array. That said, some RAID levels (the parity-based one, basically) can sometimes (and in some specific implementation) increase the write amplification factor, so this in turn means higher work for the GC routines.

• A lot of articles and discussion from earlier years concerns SLC vs MLC flash and that SLC is preferable, due to its much longer lifespan, however it seems all SSDs (regardless of where they sit on the Consumer-to-Enterprise spectrum) are MLC thesedays - is this distinction of relevance anymore

SLC drives have basically disappeared from the enterprise, being relegated mainly to military and some industrial tasks. The enterprise marked is now divided in three grades:

• HMLC/MLCe flash is the one with the better binned MLC chips, and certified to sustain at least 25000/30000 rewrite cycles;
• 3D MLC chips are rated at about 5000-10000 rewrite cycles;
• normal planar MLC and 3D TLC chips are rated at about 3000 rewrite cycles.

In reality, any of the above flash types should provide you with plenty of total write capacity and, in fact, you can find enterprise drives with all of the above flash types.

The real differentiation between enterprise and consumer drives are:

• the controller/firmware combo, with enterprise drives much harder to die due to unexpected controller bug;
• the power-protected write cache, extremely important to prevent corruptions to the Flash Translation Layer (FTL), which is stored on the flash itself.

Enterprise grade drivers are better mostly due to their controllers and power capacitors, rather than due to better flash.

• Enterprise SSDs tend to have have much higher endurance / write-limits (often measured in how many times you can completely overwrite the drive in a day, throughout a drive's expected 5 year lifespan), does this obviate any concerns over Write-Amplification caused by not running TRIM?

As stated above, enterprise grade drives have much higher default spare space (~20%) which, in turn, drastically lowers the need for regular TRIMs

Anyway, as a side note, please consider some software RAIDs that support TRIMs (someone said Linux MDRAID?)