Am Thu, 14 Sep 2017 16:45:25 +0200 schrieb Coly Li <i@xxxxxxx>: > 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. I could imagine that one way want to use a fast, more expensive disk as read cache, while using a smaller SLC SSD as write cache for better longevity and reliability. Because: When you write cache SSD breaks, things go really bad. If you read cache breaks: No problem, it just slows down. So, in conclusion: The recommendation may not be because of performance... Better performance may just be a (small) side effect. > > > > On 14/09/2017 04:58, Coly Li wrote: > >> On 2017/9/11 下午4:04, FERNANDO FREDIANI wrote: > [...] > >> 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. > -- Regards, Kai Replies to list-only preferred. -- 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
