FlashSSD for separated ZIL
I´ve reported about the separated ZIL a few days ago. The problem of the described NVRAM PCI card is, that you can´t do a clusterfailover with such a device. How do you want to failover the seperate log, when the log is on a card in the failed server? Sun had a product called Prestoserve, that was used to accelerate NFS and DB. It was static RAM with a battery. It was great for benchmarks, but suffered by the cluster problem. Thus you should use some external device, that can failover with the rest of your storage. The obvious choice would be a RAM-based Solid State Disk(SSD). But these are quite expensive: You need the RAM, you need a harddisk to keep the data persistent when power fails, and you need a rechargeable battery or an capacitor that´s able to power the SSD until all data is written to the hard disk. A Flash-Based SSD would be a more sensible choice, as Flash is a non-volatile memory by nature. Such a disk costs you approximatly 400$. But most people think “Oh no, wear will destroy it within a few days”. Experiences with el-cheapo CF-cards underline this assumption. But let´s calculate with the specifications of a leading brand flash disk. Let´s assume: A 32 GB flash-based SSD is specified for 2.000.000 write cycles. We have a sustained stream of 40 MB per second (conservative assumption). The wear leveling is perfect (perhaps supported by a seperate ZIL algorithm, that looks at the flash SSD as a cyclic buffer). Okay, a little math:
So this flash SSD wouldn´t fail by wear within the usable live of the storage and the server, even when you write 40 MB every second to it. I´m sure, that a flash disk doesn´t run such a long time, but this is not a wear problem, it´s the problem, that modern electronic hasn´t the build quality of former times. Based on this considerations, a flash SSD would be an interesting choice for the separated ZIL. Or at least: Wear isn´t a reason for not using Flash SSD PS: There is one point, i´m not perfectly sure, but i interpret the 2 million write cycles as the ability to erase and write the full disk 2 million times.