Question: The limited writings and capacityprice of the SSD are known drawbacks as the sensitivity to shocks of the HDD.
The hybrid SSHD (standard HDD+SSD cache) have a classic HD combined with a small SSD used as cache and managed by the firmware of the SSHD.
Now I want to know what happens when the SSD cache reaches the writing limit quote for all the cells, the two possible alternatives are:
a) simply the firmware stop to use the SSD caching and the SSHD becomes a standard HDD
b) the SSHD becomes unusable
What is the right answer? (The b alternative would make the SSHD choice, the less durable alternative and the worst choice ever for a server.)
I have searched reliable sources about this, but I haven’t found anything.
The mechanical problems of HDD are very rare if they they are not subjected to shocks during the readwrite operations, in standard condition the MTF range between 1 million to 1.5 million hours for a modern HDD. In the SSD, particularly SSD TLC, wearing is a problem, MLC typical maximum PE-cycle-per-block numbers range from 1500 to 10,000 (5000 mostly). Reach 5000 cycles is relatively easy if the storage is used intensively (especially in server use). So It’s really important the SSD cache durability, and in addition must be considered also that cache-SSD sectors in SSHD are used even most intensively of a standard SSD that can distribute the wearing on a larger space.
Answer: Cache
+David Schwartz, when you mentioned about it’s already in the OS cache, I cannot agreed any more.
But the problems is size matter, if the file cache managed by operating system is smaller than the size of SSD cache, the SSD cache in SSHD can still save your time to read from disk.
In my situation, I am running OpenSuSE 12.3 x64 on Lenovo ThinkCentre Edge 72z with 16GB of RAM. My file cache is about 3GB after 14 hours. If your computer is 64GB of RAM, the file cache might be more than 8GB. As mentioned above, the 8Gb SSD cache is less useful than file cache. That’s why Seagate provides Seagate Enterprise Turbo SSHD with 32GB SSD cache.
Wear Out
Toshiba provides a faq to explain what shall happen when SSD cache wear out. It should be function as a normal hard drive.
Conculsion
Before the SSD cache wear out, there will be more and more damaged chips in SSD which means the available SSD cache is becoming smaller and smaller. User shall notice that the performance is decline slowly without any warning from S.M.A.R.T.
You may check my SSHD will function as normal Hard Drive when the SSD Cache wear out for detail and my update on this matter.