On Mon, Mar 27, 2017 at 03:20:37PM +0200, Christian Theune wrote: > Hi, > > > On Mar 27, 2017, at 3:07 PM, Hugo Mills <h...@carfax.org.uk> wrote: > > > > On my hardware (consumer HDDs and SATA, RAID-1 over 6 devices), it > > takes about a minute to move 1 GiB of data. At that rate, it would > > take 1000 minutes (or about 16 hours) to move 1 TiB of data. > > > > However, there are cases where some items of data can take *much* > > longer to move. The biggest of these is when you have lots of > > snapshots. When that happens, some (but not all) of the metadata can > > take a very long time. In my case, with a couple of hundred snapshots, > > some metadata chunks take 4+ hours to move.
> Thanks for that info. The 1min per 1GiB is what I saw too - the “it > can take longer” wasn’t really explainable to me. > As I’m not using snapshots: would large files (100+gb) with long > chains of CoW history (specifically reflink copies) also hurt? Yes, that's the same issue -- it's to do with the number of times an extent is shared. Snapshots are one way of creating that sharing, reflinks are another. > Something I’d like to verify: does having traffic on the volume have > the potential to delay this infinitely? I.e. does the system write > to any segments that we’re trying to free so it may have to work on > the same chunk over and over again? If not, then this means it’s > just slow and we’re looking forward to about 2 months worth of time > shrinking this volume. (And then again on the next bigger server > probably about 3-4 months). I don't know. I would hope not, but I simply don't know enough about the internal algorithms for that. Maybe someone else can confirm? > (Background info: we’re migrating large volumes from btrfs to xfs > and can only do this step by step: copying some data, shrinking the > btrfs volume, extending the xfs volume, rinse repeat. If someone > should have any suggestions to speed this up and not having to think > in terms of _months_ then I’m all ears.) All I can suggest is to move some unused data off the volume and do it in fewer larger steps. Sorry. Hugo. -- Hugo Mills | Jenkins! Chap with the wings there! Five rounds hugo@... carfax.org.uk | rapid! http://carfax.org.uk/ | Brigadier Alistair Lethbridge-Stewart PGP: E2AB1DE4 | Dr Who and the Daemons
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