Fernando-- I don't think it really matters. Before, when capacities of SSD were really small and endurance was a big concern it made sense to have a separate write cache made out of SLC flash-- now, being able to wear-level over an entire large MLC device is where the longevity comes from. So I understand why ZFS made the tradeoffs they did (also the read path / write path functionality were added at different times by different people)-- but I don't think you'd make the same choices in implementation today. As Coly points out, there's a small benefit to having different redundancy policies-- you don't need RAID-1 for read cache because it's not too big of a deal if you lose it. But handling this properly-- having multiple cache devices and ensuring that not-dirty data has only one copy and dirty data has multiple copies-- is fairly complicated for various reasons. And having separate devices IMO is not a good idea today-- it's both complicated to deploy and means that you concentrate most of the writes to one disk (e.g. you don't wear-level over all of the disk capacity). Mike On Tue, Sep 26, 2017 at 12:28 PM, FERNANDO FREDIANI <fernando.frediani@xxxxxxx> wrote: > Hello > > Has anyone had any consideration about the usage of a single SSD for both > read and write and how that impacts the overall performance and drive's > endurance ? > > I am interested to find out more in order to adjust the necessary stuff and > monitor it accordingly. > > Fernando > > > > On 14/09/2017 12:14, FERNANDO FREDIANI wrote: >> >> Hello Coly >> >> I didn't start this thread to provide numbers but to ask people view >> on the concept and compare how flash technology works compared to how >> it used to be a few years ago and I used ZFS case as an example >> because people used to recommend to have separate devices until >> sometime ago. My aim is to understand why this is not the >> recommendation for bcache, if it already took in consideration newer >> technology or if has anything else different on the way it deals with >> write and read cache. >> >> Regards, >> Fernando >> >> >> On 14/09/2017 12:04, Coly Li wrote: >> >> On 2017/9/14 下午4:54, FERNANDO FREDIANI wrote: >> >> It depends on every scenario. SSDs generally have a max throughput and >> a max IOPS for read and write, but when you mix them it becomes more >> difficult to measure. A typical SSDs caching device used for both >> tasks will have the normal writing for doing the writeback caching, >> have writes coming from the permanent storage to cache content more >> popular (so to populate the cache) and will have reads to serve >> content already cache to the user who requested. >> >> Another point perhaps even more important than that is how the SSD in >> question will stand for wearing. Now a days SSDs are much more >> durable, specially those with higher DWPD. I read recently that newer >> memory technology will do well compared to previous ones. >> >> Hi Fernando, >> >> It will be great if you may provide some performance numbers on ZFS (I >> assume it should be ZFS since you mentioned it). I can understand the >> concept, but real performance number should be more attractive for this >> discussion :-) >> >> Thanks in advance. >> >> Coly Li >> >> On 14/09/2017 11:45, Coly Li wrote: >> >> On 2017/9/14 下午3:10, FERNANDO FREDIANI wrote: >> >> Hello Coly. >> >> If the users reads a piece of data that is just writen to SSD (unlikely) >> it should first and in any condition be commited to the permanent >> storage and then read from there and cached in another area of the SSD. >> Writaback cache is very volatile and lasts only a few seconds while the >> data is not yet committed to permanent storage. >> >> In fact multiple device suport is not implemented yet, that's why I am >> asking it and comparing with other well technology as ZFS. >> >> Hi Fernando, >> >> Do you have some performance number to compare combined and separated >> configurations on ZFS ? If the performance improvement is not from >> adding one more SSD device, I don't why dedicate read/write SSDs may >> help for performance. In my understanding, if any of the SSD has spared >> throughput capability for read or write, mixed them together on both >> SSDs may have better performance number. >> >> >> Coly Li >> >> >> On 14/09/2017 04:58, Coly Li wrote: >> >> On 2017/9/11 下午4:04, FERNANDO FREDIANI wrote: >> >> Hi folks >> >> In Bcache people normally use a single SSD for both Read and Write >> cache. This seems to work pretty well, at least for the load we have >> been using here. >> >> However in other environments, specially on ZFS people tend to suggest >> to use dedicated SSDs for Write (ZIL) and for Read (L2ARC). Some say >> that performance will be much better in this way and mainly say they >> have different wearing levels. >> The issue now a days is that SSDs for Write Cache (or Writeback) don't >> need to have much space available (8GB normally is more than enough), >> just enough for the time until data is committed to the pool (or >> slower disks) so it is hard to find a suitable SSD to dedicate to this >> propose only without overprovisioning that part. >> On the top of that newer SSDs have changed a lot in recent times using >> different types of memory technologies which tend to be much durable. >> >> Given that I personally see that using a single SSD for both Write and >> Read cache, in any scenarios doesn't impose any significant loss to >> the storage, given you use new technology SSDs and that you will >> hardly saturate it most of the time. Does anyone agree or disagree >> with that ? >> >> Hi Fernando, >> >> If there is any real performance number, it will be much easier to >> response this idea. What confuses me is, if user reads a data block >> which is just written to SSD, what is the benefit for the separated SSDs. >> >> Yes I agree with you that some times a single SSD as cache device is >> inefficient. Multiple cache device on bcache is a not-implemented yet >> feature as I know. >> >> Thanks. >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-bcache" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-bcache" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-bcache" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
