> The thing is- as far as I know the OS doesn't ask the disk to find a place > to fit the data. Instead the OS tracks what space on the disk is free and > then tells the disk where to write the data.
Yes and no, I did not formulate my idea clearly enough, sorry for confusion ;) Yes - The disks don't care about free blocks at all. For them they are just LBA sector numbers. No - The OS does track which sectors correlate to its logical blocks it deems suitable for a write, and asks the disk to position its mechanical head to a specific track and access a specific sector. This is a slow operation which can only be done about 180-250 times per second for very random I/Os (may be more with HDD/Controller caching, queuing and faster spindles). I'm afraid that seeking to very dispersed metadata blocks, such as traversing the tree during a scrub on a fragmented drive, may qualify as a very random I/O. This reminds me of a long-hanging "BP Rewrite" project which would allow live re-arranging of ZFS data allowing, in particular, some extent of defragmentation. More useful usages would be changes to RAIDZ levels and number of disks though, maybe even removal of top-level VDEVs from a sufficiently empty pool... Hopefully the Illumos team or some other developers would push this idea into reality ;) There was a good tip from Jim Litchfield regarding VDEV Queue Sizing, though. Possible current default for zfs_vdev_max_pending is 10, which is okay (or may be even too much) for individual drives, but is not very much for arrays of many disks hidden behind a smart controller with its own caching and queuing, be it a SAN box controller or a PCI one which would intercept and reinterpret your ZFS's calls. So maybe this is indeed a bottleneck - which you would see in "iostat -Xn 1" as "actv" field numbers which are near the configured queue size. //Jim -- This message posted from opensolaris.org _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
