5 ms·
What's the lifetime of an optane or NVMe drive when used as a constantly thrashing cache? Weeks? Months? Edit: Missed this the first time through: > Calculati
by bradleyjg 8y ago
What's the lifetime of an optane or NVMe drive when used as a constantly thrashing cache? Weeks? Months?
Edit: Missed this the first time through:
> Calculating this is done by monitoring an SSD's tolernace of "Drive Writes Per Day". If a 1TB device could survive 5 years with 2TB of writes per 24 hours it has a tolerance of 2 DWPD. Optane has a high tolerance at 30DWPD, while a high end flash drive is 3-6DWPD.
- nisa 8y agodatasheet says something along 5pb write endurance.
- ec109685 8y agoThey also talk in the paper of keeping the thrashing parts of the cache in RAM. Facebook for example calculates the effective hit rate of a larger set of items and only caches those that won’t be quickly purged out / overwritten in secondary storage.
- ebikelaw 8y agoIt's only four more years before the patent on adaptive replacement caching expires. Then we can use it in memcached ...
- ksec 8y agoSerious ? Doesn't ZFS already uses it?
- ebikelaw 8y agoSun licenses the patent and has related patents on the same thing. Sun provides an implementation under the CDDL license. ZFS on Linux is distributed under the CDDL. Linux is distributed under the GPL for which no patent holders have granted permission to use the patented inventions. Many other implementations exist including one under the Apache license and one under the Mozilla license that I found in two seconds on github. The whole thing is a mess.
- IcePic 8y agoI think there's only a part of ARC that was patented so if you ran the lists differently, you could basically use the double-list idea where you move entities back and forth between them as long as the lists aren't of the same kind.
- logophobia 8y agoI basically used a memory-weighted variant of the ARC algorithm, sufficiently different so that the patent doesn't cover it. I imagine there are quite a couple of variants of ARC you can use without violating the patent.
- zaarn 8y agoThere is CAR which has almost the same performance and no patent. You can use that.
- eternalban 8y agoWhy not just use LRU?
- Symmetry 8y agoPerformance. The Wikipedia page on cache policies is pretty good. https://en.wikipedia.org/wiki/Cache_replacement_policies#Clock_with_adaptive_replacement_(CAR) https://en.wikipedia.org/wiki/Cache_replacement_policies#Clo...
- eternalban 8y agoWiki says "Substantially" better than LRU but actual results seem to show performance converging to same levels the larger the cache gets. (See page 5.) https://dbs.uni-leipzig.de/file/ARC.pdf https://dbs.uni-leipzig.de/file/ARC.pdf [p.s. there is also the matter of the (patterns in the) various trace runs. Does anyone know where these traces can be obtained?]
- ebikelaw 8y agoAll caches have equal hit rates in the limit when the size of the cache approaches infinity. For finite caches, ARC often wins. In practical experience I've found that a weighted ARC dramatically outperformed LRU for DNS RR caching, in terms of both hit rate and raw CPU time spent per access. This is because it was easy to code an ARC cache that had lock-free access to frequently referenced items; once an item had been promoted to T2 no locks were needed for most accesses. With LRU it's necessary to have exclusive access to the entire cache in order to evict something and add something else. Of course there are more schemes than just LRU and ARC, and one can try to employ lock-free schemes more than I'm willing to do. This is just my experience.
- ec109685 8y agoThat’s unfortunate. It’s a natural architecture you stumble into when you have a two tier cache (was doing this 10 years ago when we had a memory mapped cache and a secondary spinning disk cache for a popular website).
- NovaX 8y agoThankfully there are better algorithms that are not encumbered. ARC is fairly memory hungry and requires a large cache to be effective. It is not as scan resistant and does not capture frequency as well as many believe. LIRS or TinyLFU based policies are what new implementations should be based on. https://github.com/ben-manes/caffeine/wiki/Efficiency https://github.com/ben-manes/caffeine/wiki/Efficiency
- wtallis 8y agoIntel has stated that the next batch of enterprise Optane SSDs will increase the endurance rating to 60 drive writes per day, which will finally put it beyond even the historical records for drives that used SLC NAND flash.
- djsumdog 8y agoAre there current writes-per-day/durability numbers for traditional spinny disks? I can't seem to find anything other than SSD numbers.
- monocasa 8y agoSpinning rust doesn't really express durability in terms of number of writes. The two are orthogonal for that technology.
- wtallis 8y agoSome hard drives come with workload ratings. For example, the WD Gold is rated for 550TB (reads+writes) per year (if you run it 24/7). But because the wearout mechanisms are so different between hard drives and SSDs, you can't make a very meaningful comparison between them.
- dragontamer 8y agoHard Drives don't fail that way. Hard drives fail in other ways, so "writes per day" is simply irrelevant to the hard drive market. Hard Drives fail because of vibration, broken motors, and things like that. MTBF is the typical metric for hard drives. There are also errors that pop up if data sits still too long (on the order of years), because the magnetic field loses its charge over time.
- dormando 8y agoMuch of the items that will end up on an SSD are already long lived. Every gigabyte of RAM runway before flushing items reduces the write load, since recent items are at highest risk of being overwritten/deleted.. also stuff with shorter TTL's won't be persisted (or can be persisted to bucketed pages for avoiding being compacted). TL;DR: there's a lot to it and I'll be going into it in future posts. The full extstore docs explain in a lot of detail too.