Hybrid Drives and FreeNAS/ZFS?

[DaPlumber]

Does anyone have any thoughts, or preferably some hard data on how well (or not) FreeNAS does with "hybrid" drives. I'm talking about the HDD's that have a smallish, typically 4GB or 8GB flash cache built in to the drive and functionally transparent to the SATA usage.

I suspect that they will have a small effect on data that "fits" within the cache and just like in desktop usage they'll benefit reads almost exclusively.

If this has been discussed before I couldn't find it and a link would be appreciated...

 

[cyberjock]

It hasn't been discussed, but I wouldn't spend money on hybrid drives. It's a pretty well known fact that the hybrid drives were suffering from major problems akin to what SSD drives were going through when they first hit the market. Lots of lost data from firmware bugs and such. I've never owned one but friends I've talked to that tried to get me to buy them and then bought them themselves were not happy users.

I'd think that the hybrid drives would be a major disappointment. Many of them also require you install software on your machine to help manage what is stored on the flash memory. I've never seen any of them that were for FreeBSD so I think that's a non starter.

Even all of that aside, I wouldn't expect much. Buying 2TB drives and getting 8GB of flash isn't going to provide a significant benefit, and if those firmware bugs exist there's a high propensity to have "bad days" with your pool.

Sorry, but I'll stick with tried and true "antique" platter based drives until SSDs mature and are cost effective.

 

[Ericloewe]

I'd expect that puny flash cache to end up containing very similar data to the ARC, which makes it pretty much redundant...

There's also a model from WD that has a larger SSD, but it's the biggest kludge storage has seen since SATA port multipliers. Instead of going with a SATA port multiplier (for understandable reasons) to connect two drives to one port, the drive's controller assigns the first part of the drive's address space to the SSD, with the rest being HDD. You can imagine just how much pain this setup is sure to bring...

 

[DaPlumber]

Oh yeah, that WD drive is Windows only because of drivers. Even were I a Windows user that kind of custom driver for a fundamental function (boot disk) would have me running screaming for the hills. I was talking about the drives where it's as transparent as the RAM cache on the HDD controller. Poster child is Seagate's "SSHD".

My thoughts were the same that unless the Flash is larger than the ARC on the system it's basically going to have much the same effect as L2ARC but without the ZFS overhead. I.e. it could wind up slower for most I/O loads, with maybe iSCSI being the exception.

@cyberjock: I agree, I am continually disappointed at how many people don't know that for "massive" sequential I/O (hello backups, media!) most "spinning rust" is faster than most "dirty sand". The hybrids *DO* make a case for system disks where a SSD read cache can accelerate a workload that includes a lot of small random I/O's. Being able to spin down and park a laptop drive more often is also a good thing. "Memristors" to the rescue? I see those as a new generation of SSD long before we start changing to an all memristor architecture. ;-) Then again I've been disappointed with magnetic-bubble and holographic memory before... ;)

What prompted all this is I discovered I have two Seagate 1TB 2.5" hybrids and I got to wondering...

 

[Dennis.kulmosen]

Well if you have them laying around, put them to the test. ;-)
I am really curious to see the performance of the SSHD. My concerns is about the wear on the SSD parts.

 

[DaPlumber]

Well if you have them laying around, put them to the test. ;-)
I am really curious to see the performance of the SSHD. My concerns is about the wear on the SSD parts.

Yeah, about that...

It's added to my extensive "to-do" list. ;) Unfortunately it's kinda way down on the list. :(

 

[Savage]

That WD model (something-something black edition) w/ the larger SSD is a windows only device if I recall. It requires a driver specific to windows to see both the SSD and the HDD at the same time. The product was positioned use in windows laptops w/ a single 2.5 drive slot, basically.

As for SSHDs with ZFS? There have been some concerns about the longevity of the NAND cache on the drives. One possible issue there. I'm not trying to spread FUD but the manufacturers aren't exactly blasting lifespan specs about that w/ their product info. A pool of drives w/ constant active usage may exceed the writes/day that the flash was designed for. These products seem to be intended for use as boot drives in consumer laptops/desktops after all. Failure conditions aren't exactly well documented either vs spinning disk drives.

I would love to know more about the caching schemes used. My very limited benchmark tinkering on the 1TB models showed write throughput in the 120-130MB/s range and IOPs consistent w/ a 7200 spinning disk drive. Read was roughly double or slightly more for anything in the cache and back down in the spinning disk range for anything not hot.

I would hedge a bet that an SSHD backed pool would see some speed hikes in a read heavy workload. But why bother? Dump a big L2ARC on your pool and be done with it.

I also couldn't speak to the corruption issues that cyberjock mentions. The 3.5 inch form factor 1TB seagate SSHD drives I have deployed in a great many workstations have been totally trouble free; we purchased ~30 of those drives ~1 month after they became available.