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> That said, if you're careful then that predictability should be a good thing. Yes, it's a very good thing. In a high end SSD storage system, you predict earl
by codemac 14y ago
> That said, if you're careful then that predictability should be a good thing.
Yes, it's a very good thing. In a high end SSD storage system, you predict early enough based on a calculation of how many drives there are, and what their current wear is, what type they are (SLC, eMLC, cMLC), etc. Then you phone home and have a drive delivered before the user even sees a disk failure.
With HDD's, the failure rate is so random that the disk completely failing is the signal that get's a replacement drive into the enclosure. S.M.A.R.T-type alert systems have been epic failures (too little info too late). The difference is because of the RAID rebuild having a lower probability of failure (let's set aside mulitple URE's for a second) that you can count on the mttf of the next drive failing being longer than the time it takes to get a drive out there.
However this is not much of a guarantee, so most people crazy over provision their storage.
SSDs let you predict this, thus provision correctly, and choose how to replace the drives to least impact the customer. It's win win to have predictable failure. I don't understand people who say otherwise.
- bbulkow 14y agoIt's hard to predict without SMART features to measure the current wear state. Write amplification from the file system, and from the drive itself if you're not using large block writes, means you can't just calculate - you have to measure.
- moe 14y agoTo extend on your comment: It's not possible to predict individual drive failure with reasonable accuracy. It's disconcerting to see the parent comment suggesting this still sits at the top of the thread. You can roughly predict the longest possible lifespan for a SSD under a given workload. Regardless, a significant percentage of drives will still die earlier than that.
- codemac 14y agoI'm only saying this from experience developing storage systems that are yet unreleased. You can predict the lifespan of the SSD in the storage system if you give up many of the functions of the SSD controller and put them in software RAID. If you're talking about most incredibly naive SSD storage systems available today (excluding violin memory and maybe xtreme/pure), then I agree with you.
- peschkaj 14y agoDepending on the SSD vendor, many drives expose performance counters to help you estimate wear level.
- codemac 14y agoI'm sorry, I didn't mean you'd calculate some value once for all the drives. It's definitely something you measure over the lifespan of the SSD itself with the rest of your QoS subsystem. Reads, program/erases, controller ecc/ read disturb management, the g/p list mapping of the blocks... This all has to be taken into account in a dynamic way. And yes, some people are doing this at a higher level than the SSD controller.