On Wed, Apr 16, 2008 at 3:19 PM, Richard Elling <[EMAIL PROTECTED]> wrote: > Brandon High wrote: > > The stripe size will be across all vdevs that have space. For each > > stripe written, more data will land on the empty vdev. Once the > > previously existing vdevs fill up, writes will go to the new vdev. > > In general, for the first time a device is filled, space will be allocated > in the spacemap one slab at a time. The default slab size is 1MByte. > So when you look at physical I/O, you may see something like 8 > 128kByte sequential writes to one vdev concurrent with 8 128kByte > sequential writes to another vdev, and so on. Reads go where > needed.
In a case where a new vdev is added to an almost full zpool, more of the writes should land on the empty device though, right? So maybe 2 slabs will land on the new vdev for every one that goes to an previously existing vdev. One problem that I'm having trouble getting my head around, and I'm sure other are too, is that in terms of block allocation, zfs's dynamic striping is absolutely nothing like raid-0. Correct me if I'm wrong, but while the end result is similar (i/o is distributed against all vdevs in the pool) the details are very different. That it's compared to raid-0 just makes matters worse due to terminology overlap and a preconception of how striping works. Dynamic striping doesn't really seem to be striping at all - It's dynamically distributed block allocation across the members of a zdev. Likewise, raidz and raidz2 compare favorably to raid-5 and raid-6, but don't share implementation details. Some of the literature out there like Jeff's blog mentions a raidz "full stripe write". This is not really correct, since it isn't writing a full stripe due to the way the storage is allocated. The end result is equivalent to a full stripe write in a conventional parity protected system, but it's not the same thing. Short of creating a whole new dialect around zfs, I can't think of a way to eliminate the overlap but it is a little confusing when still figuring out how things work. -B -- Brandon High [EMAIL PROTECTED] "The good is the enemy of the best." - Nietzsche _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
