> On Apr 13, 2016, at 4:40 PM, Daniel Carosone <[email protected]> > wrote: > > Yes, agreed and understood. It is a space reservation that ensures some > number of blocks will never be allocated. > > That's not exactly the same as them never being used, due to CoW updates, but > it's very close. Once the pool is close to full, any writes that don't > immediately free the original blocks will get denied. > > The net effect is the same: a relatively constant number of free blocks for > the ssd controller to use in its own wear levelling and performance > management. Overprovisioned storage with lots of spare blocks above whatever > the device keeps internally already. > > At least, it seems so to me. My question, elaborated thus, is: what is the > difference you see that makes it insufficient? > > Oh, are we not issuing TRIM from zfs as space is freed? > no > That would explain it. If so, writing zeros into the reserved space (without > compression, dedup, or snapshots) occasionally will tell the ssd controller > the blocks are empty. > > I feel this is an effective workaround entirely within zfs, without resorting > to the ugly tricks of multiple partitioning schemes and inflexible external > allocations we both dislike. > >
pedantic question: why not buy good quality SSDs? In my studies, good quality SSDs with decent overprovisioning perform more consistently than el-cheapos. FWIW, the preponderance of the evidence suggests that wear out is not as important as age. COW file systems like ZFS are particularly well behaved. https://www.usenix.org/conference/fast16/technical-sessions/presentation/schroeder <https://www.usenix.org/conference/fast16/technical-sessions/presentation/schroeder> https://users.ece.cmu.edu/~omutlu/pub/flash-memory-failures-in-the-field-at-facebook_sigmetrics15.pdf <https://users.ece.cmu.edu/~omutlu/pub/flash-memory-failures-in-the-field-at-facebook_sigmetrics15.pdf> -- richard > On 13 Apr 2016 18:27, "Dirk Steinberg" <[email protected] > <mailto:[email protected]>> wrote: > Am 13.04.2016 um 09:53 schrieb Daniel Carosone <[email protected] > <mailto:[email protected]>>: >> What is wrong with a dataset with refreserv set? >> > It does not actually reserve any specific blocks on the disk (LBAs for SATA) > which would > allow the SSD controller to deduct that a certain part of the SSD is not > being used. > > freservation is purely a (virtual) space accounting method of ZFS. > > smartos-discuss | Archives > <https://www.listbox.com/member/archive/184463/=now> > <https://www.listbox.com/member/archive/rss/184463/21953302-fd56db47> | > Modify <https://www.listbox.com/member/?&;> Your Subscription > <http://www.listbox.com/> ------------------------------------------- smartos-discuss Archives: https://www.listbox.com/member/archive/184463/=now RSS Feed: https://www.listbox.com/member/archive/rss/184463/25769125-55cfbc00 Modify Your Subscription: https://www.listbox.com/member/?member_id=25769125&id_secret=25769125-7688e9fb Powered by Listbox: http://www.listbox.com
