Re: bcache & btrfs: "Btrfs detected SSD devices, enabling SSD mode"

[Rolf Fokkens <rolf-6w2rdlBuEQTpMFipWq+H6g@public.gmane.org>]

On 09/20/2013 06:25 PM, Gabriel de Perthuis wrote:
> SSD […] optimizations (e.g. avoiding unnecessary seek optimizations,
> sending writes in clusters, even if they are from unrelated files. This
> results in larger write operations and faster write throughput)
>
> The optimisations should be beneficial iff bcache is used in writeback
> mode.  ssd_spread and discard are enabled separately; I'm not sure what
> ssd_spread (from the wiki: more strict about finding a large unused
> region of the disk for new allocations) would do for bcache.
 From observing iostat I have the impression that bcache sometimes 
bypasses the cache, probably related to sequential_cutoff. Seems like 
btrfs tries to cluster writes (because an SSD is detected) and next 
bcache bypasses the SSD (because the writes are clustered). Doesn't seem 
the right kind of cooperation :-)

(Assuming of course that clustered writes are based on sequential writes)