On 2017/9/17 上午11:43, Nix wrote: > On 9 Sep 2017, Coly Li spake thusly: > >> When bcache does read I/Os, for example in writeback or writethrough mode, >> if a read request on cache device is failed, bcache will try to recovery >> the request by reading from cached device. If the data on cached device is >> not synced with cache device, then requester will get a staled data. >> >> For critical storage system like database, recovery from staled data may >> result an application level data corruption, which is unacceptible. But >> for some other situation like multi-media stream cache, continuous service >> may be more important and it is acceptible to fetch a staled chunk of data. >> >> This patch tries to solve the above conflict by adding a sysfs option >> /sys/block/bcache<idx>/bcache/recovery_from_staled_data >> which is defaultly cleared (to 0) as disabled. Now people can make choices >> for different situations. > > 'Staled' is not a word, though perhaps it should be. You probably want > to call it recovery_from_stale_data. But given the description below... > Hi Nix, Thanks for pointing out this. Sure, I will replace all typos with 'stale'. Good suggestion :-) >> With this patch, for a failed read request in writeback or writethrough >> mode, recovery a recoverable read request only happens in one of the >> following conditions, >> - dc->has_dirty is zero. It means all data on cache device is synced to >> cached device, the recoveried data is up-to-date. >> - dc->has_dirty is non-zero, and dc->recovery_from_staled_data is set >> to 1. It means there is dirty data not synced to cached device yet, but >> option recovery_from_staled_data is set, receiving staled data is >> explicitly acceptible for requester. > > ... this name is also unclear. It sounded to me like it was an option > that recovers *from* stale data (as if the stale data was a problem to > recover from), not an option that uses stale data to *allow* recovery. > > Perhaps, instead, something like stale_data_permitted or > allow_stale_data_on_failure would be better? > I didn't notice such a confusion without your suggestion, thank you! allow_stale_data_on_failure is good for me. I will send out another patch, fix 'stale' and rename sysfs entry to allow_stale_data_on_failure.
