On Thu, 2008-08-07 at 20:02 +0200, Andi Kleen wrote: > Chris Mason <chris.mason@xxxxxxxxxx> writes: > > > > Metadata is duplicated by default even on single spindle drives, > > Can you please say a bit how much that impacts performance? That sounds > costly. Most metadata is allocated in groups of 128k or 256k, and so most of the writes are nicely sized. The mirroring code has areas of the disk dedicated to mirror other areas. So we end up with something like this: metadata chunk A (~1GB in size) [ ......................... ] mirror of chunk A (~1GB in size) [ ......................... ] So, the mirroring turns a single large write into two large writes. Definitely not free, but always a fixed cost. I started to make some numbers of this yesterday on single spindles and discovered that my worker threads are not doing as good a job as they should be of maintaining IO ordering. I've been using an array with a writeback cache for benchmarking lately and hadn't noticed. I need to fix that, but here are some numbers on a single sata drive. The drive can do about 100MB/s streaming reads/writes. Btrfs checksumming and inline data (tail packing) are both turned on. Single process creating 30 kernel trees (2.6.27-rc2) Btrfs defaults 36MB/s Btrfs no mirror 50MB/s Ext4 defaults 59.2MB/s (much better than ext3 here) With /sys/block/sdb/queue/nr_requests at 8192 to hide my IO ordering submission problems: Btrfs defaults: 57MB/s Btrfs no mirror: 61.51MB/s -chris -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
