Flash capacity tier - SAS or SATA

[sappomannoz]

Hello,

I’m planing to add VSAN to my 14 nodes ESX cluster. I will have 2 DG per server and start with 3 capacity drives per DG.

SATA drives are quite cheaper and I‘m wondering if a SAS capacity tier would bring enough advantages to justify the price difference.

84 ”spindles“ even if SATA sould provide quite a considerable number of IOs

[TomHowarth]

it is not just about IO, but also resilience,  SATA drives are attached in a Daisy chain, where as SAS are in a ring,  I would also look as NL-SAS for capacity but pure SAS for Cache

[sappomannoz]

Hi Tom,

I don't think there is such a thing like NL-SAS for SSDs. I'm considering an AFA solution, and yes, cache will be either SAS or NVMe.

[TomHowarth]

you are correct regarding SSD and NL-SAS,  but my statement about SAS over SATA remains,  it is not just a IOPS decision point.  for me personally I would go SAS all the time

[TheBobkin]

Hello sappomanoz,

I would strongly advise going SAS/NVMe for cache-tier and SAS/NL-SAS for capacity-tier (if feasible). The reasoning for this is that SATA devices have a very shallow queue depth (32) and thus SATA devices used as cache can end up being major bottlenecks resulting in poor performance.

Old article but the design hasn't changed much at this level:

yellow-bricks.com/2014/06/09/queue-depth-matters/

Not sure what TomHowarth is on about regarding resiliency - a single capacity-tier drive failing should not cause any impact on the rest of the disks being accessible (perhaps he is thinking of JBODs not vSAN).

Bob

[sappomannoz]

Hello TheBobkin,

I have read Duncan's post, but he also says for flash devices the queue depth doesn't matter too much.

I will try to summon @depping  maybe he can add something to this thread.

With flash and dedupe a single drive failure will kill the whole DG indeed.

[TheBobkin]

Hello sappomannoz,

Misread and thought you were planing Hybrid here, fair enough.

Sure, single capacity-tier with dedupe+compression enabled will kill the whole DG but this is due to those features being configured on a per-DG level (not daisy-chaining or rings, SATA/SAS differences).

Bob

[TomHowarth]

I would also try Cormac Hogan too.