On Fri, Jan 26, 2007 at 06:08:50PM -0800, Darren Dunham wrote: > > What do you guys think about implementing 'zfs/zpool rewrite' command? > > It'll read every block older than the date when the command was executed > > and write it again (using standard ZFS COW mechanism, simlar to how > > resilvering works, but the data is read from the same disk it is written to= > > ). > > #1 How do you control I/O overhead?
The same way it is handled for scrub and resilver. > #2 Snapshot blocks are never rewritten at the moment. Most of your > suggestions seem to imply working on the "live" data, but doing that > for snapshots as well might be tricky. Good point, see below. > > 3. I created file system with huge amount of data, where most of the > > data is read-only. I change my server from intel to sparc64 machine. > > Adaptive endianess only change byte order to native on write and because > > file system is mostly read-only, it'll need to byteswap all the time. > > And here comes 'zfs rewrite'! > > It's only the metadata that is modified anyway, not the file data. I > would hope that this could be done more easily than a full tree rewrite > (and again the issue with snapshots). Also, the overhead there probably > isn't going to be very high (since the metadata will be cached in most > cases). Agreed. Probably in this case there should be rewrite-only-metadata mode. I agree the overhead is probably not high, but on the other hand, I'm quite sure there are workload, which will see the difference, eg. 'find / -name something'. > Other than that, I'm guessing something like this will be necessary to > implement disk evacuation/removal. If you have to rewrite data from one > disk to elsewhere in the pool, then rewriting the entire tree shouldn't > be much harder. How did I forget about this one?:) That's right. I belive ZFS will gain such ability at some point and rewrite functionality fits very nice here: mark the disk/mirror/raid-z as no-more-writes and start rewrite process (probably only limited to this entity). To implement such functionality there also has to be a way to migrate snapshot data, so sooner or later there will be a need for moving snapshot blocks. -- Pawel Jakub Dawidek http://www.wheel.pl [EMAIL PROTECTED] http://www.FreeBSD.org FreeBSD committer Am I Evil? Yes, I Am!
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