On Mon, Mar 20, 2017 at 4:20 PM, Nick Fisk <n...@fisk.me.uk> wrote: > Just a few corrections, hope you don't mind > > > -----Original Message----- > > From: ceph-users [mailto:ceph-users-boun...@lists.ceph.com] On Behalf Of > > Mike Lovell > > Sent: 20 March 2017 20:30 > > To: Webert de Souza Lima <webert.b...@gmail.com> > > Cc: ceph-users <ceph-users@lists.ceph.com> > > Subject: Re: [ceph-users] cephfs cache tiering - hitset > > > > i'm not an expert but here is my understanding of it. a hit_set keeps > track of > > whether or not an object was accessed during the timespan of the hit_set. > > for example, if you have a hit_set_period of 600, then the hit_set > covers a > > period of 10 minutes. the hit_set_count defines how many of the hit_sets > to > > keep a record of. setting this to a value of 12 with the 10 minute > > hit_set_period would mean that there is a record of objects accessed > over a > > 2 hour period. the min_read_recency_for_promote, and its newer > > min_write_recency_for_promote sibling, define how many of these hit_sets > > and object must be in before and object is promoted from the storage pool > > into the cache pool. if this were set to 6 with the previous examples, > it means > > that the cache tier will promote an object if that object has been > accessed at > > least once in 6 of the 12 10-minute periods. it doesn't matter how many > > times the object was used in each period and so 6 requests in one > 10-minute > > hit_set will not cause a promotion. it would be any number of access in 6 > > separate 10-minute periods over the 2 hours. > > Sort of, the recency looks at the last N most recent hitsets. So if set to > 6, then the object would have to be in all last 6 hitsets. Because of this, > during testing I found setting recency above 2 or 3 made the behavior quite > binary. If an object was hot enough, it would probably be in every hitset, > if it was only warm it would never be in enough hitsets in row. I did > experiment with X out of N promotion logic, ie must be in 3 hitsets out of > 10 non sequential. If you could find the right number to configure, you > could get improved cache behavior, but if not, then there was a large > chance it would be worse. > > For promotion I think having more hitsets probably doesn't add much value, > but they may help when it comes to determining what to flush. >
that's good to know. i just made an assumption without actually digging in to the code. do you recommend keeping the number of hitsets equal to the max of either min_read_recency_for_promote and min_write_recency_for_promote? how are the hitsets checked during flush and/or eviction? mike
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