Am Thu, 14 Sep 2017 09:58:25 +0200 schrieb Coly Li <i@xxxxxxx>: > 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 ? > > 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. Does bcache support more that one cache device in a cset? If yes, the best idea would be if one could implement to define one as read-mostly, and another ssd as write-mostly. This would make a non-strict policy which allows reading from the other device if the block is already there, or writing to the read-mostly device to update data in cache. Thoughts? -- 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
