The question of having SATA drives behind modern SAS expanders

April 25, 2021

A long time ago, when we were building our second generation of ZFS fileservers, I wound up digging into SAS and using SATA disks with SAS. With SAS, there are two ways to talk to your disks; you can directly connect each SAS port to a single disk or you can connect one SAS port to a SAS expander, which then connects (directly or indirectly) to some number of disks. At the time, talking to SATA disks through SAS expanders was pretty much not recommended, so we built both our second and our third generation fileservers using SATA disks that were directly connected to the SAS controller.

Recently we had a situation where the server chassis we needed to use for a project only offered a SAS expander backplane, with no option for directly connected disk ports. This caused me to wonder if putting SATA disks behind a modern SAS expander is still a bad idea, or if the situation had gotten better since 2013. I asked on Twitter and tried to do some Internet searches, with inconclusive results. On the Internet you can find a lot of older information (which generally says not do to it), but not a lot that seems to be from the last few years.

On Twitter, I got one somewhat positive report and one one sort of negative one. Even the positive report said that misbehaving disks could cause problems for the entire backplane (hopefully transient ones). As far as the Internet searches went, some people have reported good results even several years ago (eg), and there don't seem to be anyone popping up to say not to do it. On the other hand, my sense is that a lot of people running moderate scale storage don't see drive failures or other glitches very often, and those are often the big problem source.

My personal conclusion was the the issue is somewhat open, but it would probably go well with name brand components as long as all of our SATA drives were working fine. In some circumstances this is actually fine, if the system is mostly idle, can tolerate hiccups, or often is only really dealing with a single disk (for example our disk-based backup servers).

(Our answer for this situation was to take a deep breath and buy SAS disks. This particular project has the money to afford that.)

Written on 25 April 2021.
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Last modified: Sun Apr 25 00:34:45 2021
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