> Am 14.04.2016 um 02:39 schrieb Richard Elling > <[email protected]>: > > >> On Apr 13, 2016, at 4:40 PM, Daniel Carosone <[email protected] >> <mailto:[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?
Hmm, price? My 2TB 850 EVO cost me 530 EUR. How much would a „high quality“ SSD (say from Intel) cost? Maybe 2000 EUR? Also, availability in certain form factors (M.2) and capacities (I have never seen one of those HQ SSDs in 2 TB listed in a shop). > In my studies, good quality SSDs with > decent overprovisioning perform more consistently than el-cheapos. That is certainly true. > 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/184463/24390006-796fb66c> | > 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
