On 2017/9/14 下午5:31, Kai Krakow wrote: > 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. > > Hi Kai, It makes sense, and more flexible then bcache on top of raid1 IMHO. Thanks for the hint :-) Coly Li >>> >>> 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. -- 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
