This depends on how the write cache is implemented and where the cache is. If its on a caching controller that has a BBU then it depends on what happens when a f_sync is issued. If it forces the write to go down to the underlying devices then it could be a bad thing. With many caching controllers as the controller is the end device that you connect to at an OS level then you can get substantial performance increases by having the cache enabled. If you are directly connected to the drive as in there is no caching controller in front of it then some drives exhibit a performance degradation under bluestone due to all the F-syncs that are forcing writes down to media. Some drives don’t allow this parameter to be changed and some just ignore whatever setting you put on it. Darren > On 22 Nov 2021, at 15:42, Nico Schottelius <nico.schottelius@xxxxxxxxxxx> wrote: > > > Peter Lieven <pl@xxxxxxx> writes: >> Whatever SSD you choose, look if they support power-loss-protection and make sure you disable the write cache. > > I have read this statement multiple times now, but I am still puzzled by > the disabling write cache statement. What is wrong with having a (BBU > based) write cache in front of the SSDs / how does it decrease the > perfomance? > > As far as I can tell, this is just another buffer/cache that is added in > the write chain and thus should improve speed for small writes. For > write bursts / longer writes the cache will not play a role anymore. > > What am I missing? > > -- > Sustainable and modern Infrastructures by ungleich.ch > _______________________________________________ > ceph-users mailing list -- ceph-users@xxxxxxx > To unsubscribe send an email to ceph-users-leave@xxxxxxx _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx
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