3 ms·
If you're referring to NetApp's use of NVRAM, to take in writes, and acknowledging the IO before actually writing to disk, it is not that simple. They do NVRAM
by reacharavindh 8y ago
If you're referring to NetApp's use of NVRAM, to take in writes, and acknowledging the IO before actually writing to disk, it is not that simple. They do NVRAM mirroring to save those writes to another partner's NVRAM for HA. This is a network tax on core IO path.
If there is a mature Optane DIMM, it may not be necessary to do the network dance in IO path.
I sincerely hope no storage company acknowledges a write after just writing to volatile main memory. That's a recipe for disaster when a node goes down.
- voodootrucker 8y agoI don't know if it's SAN specific, but while waiting for XPoint, several vendors went to battery-backed DRAM. They confirm writes when in RAM, then whenever power is lost all RAM is dumped to flash, then loaded back up on boot: https://en.wikipedia.org/wiki/NVDIMM https://en.wikipedia.org/wiki/NVDIMM Fortunately XPoint should solve this.
- atomicUpdate 8y ago> I sincerely hope no storage company acknowledges a write after just writing to volatile main memory. That's a recipe for disaster when a node goes down. Sorry to break this to you, but this is exactly what every major storage product does. Data will be mirrored to separate DDR behind and then good status is given to the host. The data will be destaged at some point later when cache space is required for some other operation. The data itself is safe as long as the battery backup (or capacitor for smaller systems) is charged enough to handle a power outage. The storage system knows the battery levels and may not allow a write cache if there isn't enough supplemental power to destage the full write cache in the event of a power loss.
- heavenlyblue 8y agoThis is just mixing up the issues, same as saying "what if the HDD that acknowledged the write suddenly catastrophically failed"?