Ok, cool. Thanks for clearing that up. on-the-fly changing of raid-level sounds very useful. So, the way you describe it, it should also (eventually) be possible to change the raid-level on a per-file or per-directory basis? It might be quite useful to have the majority of data on raid5/raid10 but have some "scratch dirs" available with very high performance (raid0), without having to create a new filesystem and deciding how big it needs to be. On Fri, Jul 16, 2010 at 1:19 AM, Sean Bartell <wingedtachikoma@xxxxxxxxx> wrote: > On Thu, Jul 15, 2010 at 10:29:07AM +0200, Mathijs Kwik wrote: >> Hi all, >> >> I read that btrfs - in a raid mode - does not mimic the behavior of >> traditional (hw/sw) raid. >> After writing to a btrfs raid filesystem, data will only be >> distributed the way you expect after running a rebalance. > > This is not the case. When you create a btrfs filesystem with RAID > enabled, stuff written from then on will be written just like with > traditional RAID. > > The difference with traditional RAID is that different parts of the FS > can have different RAID settings. Btrfs reserves space in ~1GiB "block > groups" for data or metadata, each of which has its own RAID settings. > If you change the RAID mode for an existing filesystem (not yet > supported IIUC) or add/remove devices, the existing block groups will > keep their old RAID settings if at all possible. > > Rebalancing essentially moves everything into new block groups, which > will use the new RAID settings and be more balanced between data and > metadata. It isn't useful unless you change RAID settings, add/remove > devices, or have too much space reserved for either data or metadata. > >> [...] > >> If you never rebalance manually, will the filesystem do this in the >> background (when idle)? >> Or will the fs never rebalance itself and only become "more balanced" >> again after writing/changing some files, which it will then place on >> the drive which has the lowest balance? > > Rebalancing isn't done automatically, and nothing can become "more > balanced" until new block groups are created when you run out of space > in the old ones. > >> Basically, I'm not sure I fully understood balancing, so any info on >> this would be great. >> In traditional raid0 and raid10 (block based), it is guaranteed that >> any big file will always be stiped between disks equally, so a certain >> performance can be assumed. >> With non-automatic balancing, I'm afraid some files might not be >> distributed as well as they could, resulting in lower performance. >> Is this an issue to be aware of, or can I safely assume that for most >> use cases the performance will roughly be the same as sw-raid? >> 2 cases I'm interested in: >> - big databases(lots of rewrites) >> - real-time video-capturing (sustained write to 1 or more big files, >> needing a guaranteed write throughput) > > If you initially create the filesystem with the right RAID settings, it > will act just like normal software RAID. Balancing only comes into play > when you start changing your mind :). > >> Any info on this or balancing in general will be greatly appreciated. > -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
