Heinz Mauelshagen wrote:
>EVMS team (Steve or Corry are the ones to ask i guess),
>Joe mentioned that you are implementing a sparse and a bbr target
>for device-mapper.
Yes, we are.
>I've got a couple of questions for you.
>1. Is it read-only or read-write sparse?
Read/Write
>1b. If it's read-write sparse it must allocate storage dynamically on
>writes kicking of a user space allocator waiting for those writes
>on a table event queue, right?
No, it use the same basic COW logic as snapshotting, only there is no read.
Basically a device is allocated as a sparse target with a chunk size.
Space is reserved for a table of allocations, and as writes to
previously unused virtual chunks occur the next avaible real chunk is
allocated. This is all done in kernel space like in snapshots.
>2. I assume that the bbr target uses preallocated and inaccessable table
space
>(inaccessable in terms of regular io queued to the bbr target) to be
>allocated on the fly on io error?
Correct.
>2a. If that allocation happens, do you send an event to update metadata in
user
>space in order to reflect the relocation?
No, again we use in kernel (DM) metadata writes to record the allocation.
>2b. How do you guarantee that that update is atomic?
Locking in the kernel.
>2b. Preallocation is fine to gain performance. Additionally you guys must
be
>planning to stack bbr onto sparse (presuming read-write sparse),
right?
Ow, that hurts my head. Preallocation solves the problem of where do you
put it. We could stack bbr and sparse, but I don't see why.
Steve
