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I am well aware that flash is overprovisioned. One consumer SSDs the overprovisioning is quite small (maybe 5-10%), so short stroking the SSD will have a much

by bcrl 3y ago

I am well aware that flash is overprovisioned. One consumer SSDs the overprovisioning is quite small (maybe 5-10%), so short stroking the SSD will have a much greater effect than on enterprise SSDs. It also has the nice side effect of giving the drive more free space to work with which allows the garbage collection room to be more gradual and efficient, which can improve performance for more write intensive workloads. Actually, many storage arrays use battery backed RAM, not flash as a write buffer. Flash does not have the endurance needed to serve as the write buffer for a large storage array. Some products that use battery backed RAM for this purpose will dump the contents of that RAM onto flash. I worked on a messaging appliance that used that approach, albeit with supercaps to provide the hardware time to dump 4GB of DRAM onto a compact flash card. Supercaps were easier to monitor and maintain. There are also plenty of hardware RAID cards that use batteries for their write buffer as well. Edit: there are also persistent memories like MRAM that don't need power to retain their contents which are used in this space as well.

usernew 3y ago
yes, big arrays (symmetrix, shark) have battery backed everything - enough to take all the cache and dump it to disk when power is lost. terabytes of RAM. they also often use use a flash cache behind the RAM - that was the primary use for Optane. The reason for this, is sustained writes instead of peaks. If you let more and more sit in cache, eventually you have write folding. That results in less write IO on the backend, and you're able to have a higher sustained peak. smaller cheaper arrays (unity, pure, isilon) and HCI (nutanix, vxrail) that don't have terabytes of RAM - they have gigabytes, and pretty much always use flash as a write cache. In fact, I cannot name one that doesn't. No one in enterprise storage cares about the indurance of flash. All that means is that twice a year, a vendor engineer comes out to replace a few flash drives under your support contract. And flash has been used as a write cache behind a smaller RAM cache, for two decades, by all major storage vendors.

by bcrl 3y ago

usernew 3y ago

yes, big arrays (symmetrix, shark) have battery backed everything - enough to take all the cache and dump it to disk when power is lost. terabytes of RAM. they also often use use a flash cache behind the RAM - that was the primary use for Optane. The reason for this, is sustained writes instead of peaks. If you let more and more sit in cache, eventually you have write folding. That results in less write IO on the backend, and you're able to have a higher sustained peak. smaller cheaper arrays (unity, pure, isilon) and HCI (nutanix, vxrail) that don't have terabytes of RAM - they have gigabytes, and pretty much always use flash as a write cache. In fact, I cannot name one that doesn't. No one in enterprise storage cares about the indurance of flash. All that means is that twice a year, a vendor engineer comes out to replace a few flash drives under your support contract. And flash has been used as a write cache behind a smaller RAM cache, for two decades, by all major storage vendors.