One problem I face with the cluster as it stands now is that 2.5″ HDDs are actually quite restrictive in terms of size options.
Right now the whole shebang runs on 1TB 5400RPM Hitachi laptop drives, which so far has been fine, but now that I’ve put my old server on as a VM, that’s chewed up a big chunk of space. I can survive a single drive crash, but not two.
I can buy 2TB HDDs, WD make some and Scorptec sell them. Seagate make some bigger capacity drives, however I have a policy of not buying Seagate.
At work we built a Ceph cluster on 3TB SV35 HDDs… 6 of them to be exact. Within 9 months, the drives started failing one-by-one. At first it was just the odd drive being intermittent, then the problem got worse. They all got RMAed, all 6 of them. Since we obviously needed drives to store data on until the RMAed drives returned, we bought identically sized consumer 5400RPM Hitachi drives. Those same drives are running happily in the same cluster today, some 3 years later.
We also had one SV35 in a 3.5″ external enclosure that formed my workplace’s “disaster recovery” back-up drive. The idea being that if the place was in great peril and it was safe enough to do so, someone could just yank this drive from the rack and run. (If we didn’t, we also had truly off-site back-up NAS boxes.) That wound up failing as well before its time was due. That got replaced with one of the RMAed disks and used until the 3TB no longer sufficed.
Anyway, enough of that diversion, long story short, I don’t trust Seagate disks for 24/7 operation. I don’t see other manufacturers (other than Seagate e.g. WD, Samsung, Hitachi) making >2TB HDDs in the 2.5″ form factor. They all seem to be going SSD.
I have a Samsung 850EVO 2TB in the laptop I’m writing this on, bought a couple of years ago now, and so far, it has been reliable. The cluster also uses 120GB 850EVOs as OS drives. There’s now a 4TB version as well.
The performance would be wonderful and they’d reduce the power consumption of the cluster, however, 3 4TB SSDs would cost $2700. That’s a big investment!
The other option is to bolt on a 3.5″ HDD somehow. A DIN-rail mounted case would be ideal for this. 3.5″ high-capacity drives are much more common, and is using technology which is proven reliable and is comparatively inexpensive.
In addition, by going to bigger external drives it also means I can potentially swap out those 2.5″ HDDs for SSDs at a later date. A WD Purple (5400RPM) 4TB sells for $166. I have one of these in my desktop at work, and so far its performance there has been fine. $3 more and I can get one of the WD Red (7200RPM) 4TB drives which are intended for NAS use. $265 buys a 6TB Toshiba 7200RPM HDD. In short, I have options.
Now, mounting the drives in the rack is a problem. I could just make a shelf to sit the drive enclosures on, or I could buy a second rack and move the servers into that which would free up room for a second DIN rail for the HDDs to mount to. It’d be neat to DIN-rail mount the enclosures beside each Ceph node, but right now, there’s no room to do that.
I’d also either need to modify or scratch-make a HDD enclosure that can be DIN-rail mounted.
There’s then the thorny issue of interfacing. There are two options at my disposal: eSATA and USB3. (Thunderbolt and Firewire aren’t supported on these systems and adding a PCIe card would be tricky.)
The Supermicro motherboards I’m using have 6 SATA ports. If you’re prepared to live with reduced cable lengths, you can use a passive SATA to eSATA adaptor bracket — and this works just fine for my use case since the drives will be quite close. I will have to power down a node and cut a hole in the case to mount the bracket, but this is doable.
I haven’t tried this out yet, but I should be able to use the same type of adaptor inside the enclosure to connect the eSATA cable to the HDD. Trade-off will be further reduced cable distances, but again, they don’t need to go more than 30cm, it’ll most likely work fine.
The other interface option is USB 3.0. The motherboards have two back-panel USB 3.0 connectors and inside, two USB 3.0 ports I can potentially expose. This can be hot-plugged without changing my cluster as it stands now. The down-side is that USB incurs a greater CPU overhead than SATA.
During my migration to BlueStore, I used exactly this to provide a “temporary” OSD disk… a 1TB 7200RPM WD black in a HDD dock. The performance of that was fine, and in that case, I was willing to put up with the overhead as it was temporary.
External eSATA cases seem to be going the way of the dodo, I haven’t seen many available for sale from my usual suppliers. USB 3.0 seems to have taken over, probably because for most uses, it is “good enough”. I did ask about whether one is preferred over the other for Ceph OSD use on the Ceph mailing list, but heard nothing.
As it was, prior to undertaking the migration, I bought such a case, an el’cheapo Simplecom SE-325, along with a 4TB WD Blue for the actual drive. I was tossing up between that, and a LaCiE “Porsche” 4TB drive, but the winning factor of this was that I’d know what I was buying — the LaCiE drive could have had anything in there, manufacturers can and sometimes do substitute components in different manufacturing runs, buying the case and drive separately didn’t run that risk.
The case and drive did the job. I hooked the drive up to my laptop (I had forgotten
xhci_hcd support in the storage nodes’ kernels, which I have since fixed) and pulled a snapshot of every VM disk (Rados block device) off the Ceph cluster onto this drive as a raw disk image so I would not lose data. The drive easily kept up with the GbE link I had to the downstairs switch, and a core in the Core i5-3320M in this laptop is probably on par with the ones in the Avoton C2750s running the show.
To DIN-rail mount this, I’d need to make a cradle to take the case, and I’d need to hack some forced-ventilation into the top cover, which isn’t a difficult job. (Drill some holes, then use a nibbler tool to cut slots, then mount a small fan.)
The original PSU for this case is a 12V 2A wall wart, easily substituted with a 12V 3A LDO such as the LM1085IT-12. I may even be able to squeeze it and a heatsink into the case. I presently use one of these with the border router with a small heatsink, and so far, no problems.
If I later want eSATA, I can unscrew the original PCB and should be able to hack that in.
Short term, I can place a temporary shelf atop the battery cases and sit the HDDs there until I figure out more permanent arrangements.
Right now I’ve been battling a few health problems (sharp-eyed readers may recognise the box of “gunk” in the background which is now empty and the accompanying documentation — I’ll know more next Friday morning), and so I’ll wait until I know the outcome of those tests as there’s no point in building something grand if I’m not going to be around to enjoy it.