So what's the verdict on these SSHDs? They don't entirely seem that popular?
Weaksauce.
If you want to go that route, much better to create a Fusion Drive using an SSD and an HD. I set one up on the laptop a few months back; works great so far.
Spinners are for backup only in my professional world.
Seagate does the SSHD a disservice with over-hyping the very real benefits.
I've bought about 10 of the 4TB SSHDs, because in the maligned meme for Safari - "they're snappier".
They're not as fast as SSDs, but for lots of apps they're much faster than standard spinners.
One reason for "snappier" is the 8 GB flash read cache. Lots of frequently read stuff ends up in the SSD cache, and loads at interface speeds.
The second reason involves the 64 MiB or 128 MiB DRAM cache on the disk drive.
With most drives, that cache isn't used for writes - when the OS requests a write, the IO thread is blocked until the data is written to the disk. (This is often called "write-through" (see
http://en.wikipedia.org/wiki/Cache_(computing)#WRITE-THROUGH)
With an SSHD, the drive is always using the cache for writes. (This is usually called "write-back" caching - see the link above.) When the OS requests a write, the drive says "done" as soon as the data is in cache. Eventually, the modified data will be written to the disk. If the OS writes a sector several times, the earlier writes will be ignored - an only the last one will go to the disk.
Write-back caching means that many writes are nearly instantaneous - or at SSD speeds - even on a spinner.
Write-back caching has a huge risk though - if the power fails while the cache data is dirty, then the OS will have an inconsistent view of the file. Enterprise storage has elaborate backup schemes to ensure that the dirty cache is preserved, and will be written when power is restored. This is why drive write-back caching is disabled by default on any reasonable OS. (Even enterprise systems with battery backup for controller write caches will disable drive-level write-back caching by default.)
The Seagate SSHDs have a similar, but simpler and more elegant solution. If the power fails to the drive, the drive motor is turned into a generator (think "hybrid car") and the energy in the spinning platters is used to power logic to copy the dirty DRAM cache to the SSD part of the drive. When power is restored, the dirty data is written to the drive, and the OS sees consistency.
SSHDs aren't as fast as SSDs, but:
- they're much snappier than spinners for many apps
- they're priced at or only slightly more than equivalent NAS drives
- they're much bigger than SSDs
If so, how would I go about redirecting my user folder to the old spinner drive? And if I do this, and leave the old system on my old spinner drive... will I still be able to boot into that no problem or will it mess everything up... and also, since just the system should supposedly be on the SSD, do I just continue backing up the old spinner and not worry about backing up the system drive?