On 10/24/2017 04:01 PM, matthew patton wrote:
How in the hell is the LVM cache being used at all? It has no business caching ANYTHING on streaming reads. Hmm, it turns out dm-cache/lvmcache really is retarded. It copies data to cache on first read and furthermore doesn't appear to detect streaming reads which have no value for caching purposes.
Technically it's not entirely true to say that streaming reads have no value for caching purposes. It's conceivable to have a workload where the same file gets read over and over, in which case it might be useful to have it cached on an SSD.
As I understand it dm-cache is using smq, which essentially uses an LRU algorithm. So yes, it'll read the streaming data into the cache, but the read-once/written-never data should also be the most likely to be evicted from the cache.
For what it's worth, the Linux kernel also copies data to the page cache on reads, which is why they introduced posix_fadvise(POSIX_FADV_DONTNEED) to allow the application to indicate that it's done with the data and it can be dropped from the page cache.
Chris _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
