> That helps a lot - thank you. > I wish I knew it before... Information Roch put on his blog should be > explained both in MAN pages and ZFS Admin Guide - as this is something > one would not expect. > > It actually means raid-z is useless in many enviroments compare to > traditional raid-5.
Well, it's a trade-off. With RAID-5 you pay the RAID tax on writes; with RAID-Z you pay the tax on reads. There's also another factor at play here, which is purely a matter of implementation that we need to fix. With a RAID-Z setup, all blocks are written in RAID-Z format -- even intent log blocks, which is really stupid. If you do a lot of synchronous writes, that really hurts your write bandwidth. But it's unnecessary. Since we know that intent log blocks don't live for more than a single transaction group (which is about five seconds), there's no reason to allocate them space-efficiently. It would be far better, when allocating a B-byte intent log block in an N-disk RAID-Z group, to allocate B*N bytes but only write to one disk (or two if you want to be paranoid). This simple change should make synchronous I/O on N-way RAID-Z up to N times faster. Jeff _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
