Hello all, I understand that relatively high fragmentation is inherent to ZFS due to its COW and possible intermixing of metadata and data blocks (of which metadata path blocks are likely to expire and get freed relatively quickly).
I believe it was sometimes implied on this list that such fragmentation for "static" data can be currently combatted only by zfs send-ing existing pools data to other pools at some reserved hardware, and then clearing the original pools and sending the data back. This is time-consuming, disruptive and requires lots of extra storage idling for this task (or at best - for backup purposes). I wonder how resilvering works, namely - does it write blocks "as they were" or in an optimized (defragmented) fashion, in two usecases: 1) Resilvering from a healthy array (vdev) onto a spare drive in order to replace one of the healthy drives in the vdev; 2) Resilvering a degraded array from existing drives onto a new drive in order to repair the array and make it redundant again. Also, are these two modes different at all? I.e. if I were to ask ZFS to replace a working drive with a spare in the case (1), can I do it at all, and would its data simply be copied over, or reconstructed from other drives, or some mix of these two operations? Finally, what would the gurus say - does fragmentation pose a heavy problem on nearly-filled-up pools made of spinning HDDs (I believe so, at least judging from those performance degradation problems writing to 80+%-filled pools), and can fragmentation be effectively combatted on ZFS at all (with or without BP rewrite)? For example, can(does?) metadata live "separately" from data in some "dedicated" disk areas, while data blocks are written as contiguously as they can? Many Windows defrag programs group files into several "zones" on the disk based on their last-modify times, so that old WORM files remain defragmented for a long time. There are thus some empty areas reserved for new writes as well as for moving newly discovered WORM files to the WORM zones (free space permitting)... I wonder if this is viable with ZFS (COW and snapshots involved) when BP-rewrites are implemented? Perhaps such zoned defragmentation can be done based on block creation date (TXG number) and the knowledge that some blocks in certain order comprise at least one single file (maybe more due to clones and dedup) ;) What do you think? Thanks, //Jim Klimov _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
