George: Thanks, that was exactly what I needed. Richard: Ya, I'd noticed that.
Thanks! benr. Richard Elling wrote: > empirical look below... > > George Wilson wrote: >> Ben Rockwood wrote: >>> I've been pouring through the vdev_cache code and need help >>> understanding the cache contents. Per CR 6437054 the cache was said to >>> only cache meta-data. However, looking at the current code this no >>> longer appears to be the case. >>> >>> Can someone explain what happened here? The idea of only caching >>> metadata is odd to me because when you read-ahead you can't be certain >>> of what it is you have put in the cache until someone comes knocking >>> for >>> that block. >>> >>> As I understand the current implementation, the per-disk vdev_caches >>> are >>> 10MB LRU. Reads smaller than 16K (cache_max) are inflated to 64K >>> (cache_bshift), anything larger than 16K is therefore un-inflated. The >>> cache does not differentiate between meta-data blocks and data blocks >>> when caching. Is this correct? >>> >>> Thanks. >>> >>> benr. >>> >> Ben, >> >> The code you're looking for is in zio_read_bp_init(): >> >> if (!dmu_ot[BP_GET_TYPE(bp)].ot_metadata && BP_GET_LEVEL(bp) >> == 0) >> zio->io_flags |= ZIO_FLAG_DONT_CACHE; >> >> So any data blocks have the ZIO_FLAG_DONT_CACHE flag enabled and >> won't end up in the vdev_cache. >> > > You can also observe the kstats and note that there are far fewer > accesses than you would expect to see for any given workload. > kstat -n vdev_cache_stats > > -- richard >
