On 1 jan 2010, at 17.44, Richard Elling wrote: > On Dec 31, 2009, at 12:59 PM, Ragnar Sundblad wrote: >> Flash SSDs actually always remap new writes into a >> only-append-to-new-pages style, pretty much as ZFS does itself. >> So for a SSD there is no big difference between ZFS and >> filesystems as UFS, NTFS, HFS+ et al, on the flash level they >> all work the same. > >> The reason is that there is no way for it to rewrite single >> disk blocks, it can only fill up already erased pages of >> 512K (for example). When the old blocks get mixed with unused >> blocks (because of block rewrites, TRIM or Write Many/UNMAP), >> it needs to compact the data by copying all active blocks from >> those pages into previously erased pages, and there write the >> active data compacted/continuos. (When this happens, things tend >> to get really slow.) > > However, the quantity of small, overwritten pages is vastly different. > I am not convinced that a workload that generates few overwrites > will be penalized as much as a workload that generates a large > number of overwrites.
Zfs is not append only in itself, there will be holes from deleted files after a while, and space will have to be reclaimed sooner or later. I am not convinced that a zfs that has been in use for a while rewrites a lot less than other file systems. But maybe you are right, and if so, I agree that intuitively such a workload may be better matched to a flash based device. If you have a workload that only appends data and never changes or deletes it, zfs is probably a bit better than other file systems of not rewriting blocks. But that is a pretty special use case, and another file system could rewrite almost as little. > I think most folks here will welcome good, empirical studies, > but thus far the only one I've found is from STEC and their > disks behave very well after they've been filled and subjected > to a rewrite workload. You get what you pay for. Additional > pointers are always appreciated :-) > http://www.stec-inc.com/ssd/videos/ssdvideo1.php There certainly are big differences between the flash SSD drives out there, I wouldn't argue about that for a second! /ragge _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
