Henk Langeveld writes:
> Mario Goebbels wrote: > >>> An introduction to btrfs, from somebody who used to work on ZFS: > >>> > >>> http://www.osnews.com/story/21920/A_Short_History_of_btrfs > >> *very* interesting article.. Not sure why James didn't directly link to > >> it, but courteous of Valerie Aurora (formerly Henson) > >> > >> http://lwn.net/Articles/342892/ > > > > I'm trying to understand the argument against the SLAB allocator > > approach. If I understood correctly how BTRFS allocates space, changing > > and deleting files may just punch randomly sized holes into the disk > > layout. How's that better? > > It's an interesting article, for sure. The core of the article is actually > how a solution (b+trees with copy-on-write) found a problem (file systems). > > To answer the question, the article claims that reallocation is part of the > normal process writing data: > > > Defragmentation is an ongoing process - repacking the items > > efficiently is part of the normal code path preparing extents to be > > written to disk. Doing checksums, reference counting, and other > > assorted metadata busy-work on a per-extent basis reduces overhead > > and makes new features (such as fast reverse mapping from an extent > > to everything that references it) possible. > > It sure suggests what is happening, but I haven't got a clue on how the above > makes a difference. Translating this to the ZFS design, I guess it involves > delaying the block layout to the actual txg i/o phase, while zfs already > decides this when a block enters the txg, it's layout has been decided > already. > Henk, I don't know what 'enters the txg' exactly is but ZFS disk-block allocation is done in the ZIO pipeline at the latest possible time. > This allows for blocks to be dumped into a slog device as soon as it is > available. > I'm not sure what this refers to. -r > Cheers, > Henk > _______________________________________________ > zfs-discuss mailing list > zfs-discuss@opensolaris.org > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
