> From: cas...@holland.sun.com [mailto:cas...@holland.sun.com] On Behalf > Of casper....@sun.com > > >On Mon, 19 Apr 2010, Edward Ned Harvey wrote: > >> Improbability assessment aside, suppose you use something like the > DDRDrive > >> X1 ... Which might be more like 4G instead of 32G ... Is it even > physically > >> possible to write 4G to any device in less than 10 seconds? > Remember, to > >> achieve worst case, highest demand on ZIL log device, these would > all have > >> to be <32kbyte writes (default configuration), because larger writes > will go > >> directly to primary storage, with only the intent landing on the > ZIL. > > > >Note that ZFS always writes data in order so I believe that the > >statement "larger writes will go directly to primary storage" really > >should be "larger writes will go directly to the ZIL implemented in > >primary storage (which always exists)". Otherwise, ZFS would need to > >write a new TXG whenever a new "large" block of data appeared (which > >may be puny as far as the underlying store is concerned) in order to > >assure proper ordering. This would result in a very high TXG issue > >rate. Pool fragmentation would be increased. > > > >I am sure that someone will correct me if this is wrong. > > There's a difference between "written" and "the data is referenced by > the > uberblock". There is no need to start a new TXG when a large datablock > is written. (If the system resets, the data will be on disk but not > referenced and is lost unless the TXG it belongs to is comitted)
*Also* it turns out, what I said was not strictly correct either. I think I'm too sleepy to get this correct right now, but ... My (hopefully corrected) understanding is now: By default, all sync writes will go to ZIL entirely, regardless of size. Only if you change the ... what is it ... logbias to ... throughput. Then, if you have a large sync write, the bulk of data will be written to primary storage, while just a tiny little intent will be written to the SSD. I think I misunderstood the default. I previously thought throughput was the default, not latency. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
