Re: [RFC] Ext3 online defrag

[Eric Sandeen <sandeen@redhat.com>]

Alex Tomas wrote:
>>>>>> Eric Sandeen (ES) writes:
> 
>  ES> Alex Tomas wrote:
>  >> 3) scalable reservation
>  >> required for delayed allocation to avoid -ENOSPC at flush time.
>  >> current version uses per-sb spinlock.
> 
>  ES> Can you elaborate on this issue?  Shouldn't delayed allocation
>  ES> decrement free space immediately, and only the actual block location
>  ES> choice is delayed?  Or is this due to potential extra metadata space
>  ES> required as blocks are allocated?
> 
> exactly. in this case, reservation has nothing to do with allocation
> or preallocation of real blocks. this is just a *per-sb counter* of
> blocks reserved for allocation at flush time. it includes all
> non-allocated-yet blocks and metadata needed to allocate them (bitmaps,
> group descriptors, blocks extent tree, etc). the previous version
> of mballoc has reservation, but it doesn't scale very well being
> a single global counter protected by the spinlock. at least, in many
> regular loads I observed the reservation function in top30 of oprofile.

Thanks. XFS recently made similar scalability changes in this area, see the 2006 
OLS paper, if you're interested.

-Eric