Re: [Ext2-devel] Re: [RFC, PATCH] Reservation based ext3 preallocation

[Mingming Cao <cmm@us.ibm.com>]

On Fri, 2004-04-02 at 17:50, Andrew Morton wrote:
> hm, maybe.  We should probably also provide a per-file ext3-specific ioctl
> to allow specialised apps to manipulate the reservation size.
> 
> And we should grow the reservation size dynamically.  I've suggested that
> we double its size each time it is exhausted, up to some limit.  There may
> be better algorithms though.
You mean when the reservation window size is exhausted, right? I think
this is probably the easiest way. Maybe like the readahead window does. 
Just sometimes the window reserved does not contains much free blocks to
allocate, and we could easily reach to the upper limit.

Currently, when try to reserve a window in a block group, if there is no
window big enough for this, we skip this group and move on to the next
group.  I was thinking maybe we should keep track of the largest
avaliable reservable window when we are looking for a new window, so in
case we can't find the one with expected size, we at least could get one
within the group.

This will try to keep the file inside it's target group, and also reduce
the possibility of bogus earlier ENOSPC. Just it's a trade off:there
maybe plenty of space in the next group......who knows. What do you
think?

Also, for the the bogus earlier ENOSPC, : the filesystem probably
relatively full of reservations, so the late guy who need a new block
but don't have a reservation window will failed to allocate a block. In
this case, the easiest way as you said before is to just steal a free
block from other file's reservation window. I agree it's the extreme
case and the solution is easy, just a little concern that this will in
favor of those inodes who came first and made reservations, but whose
who come in later need a new block, every time has to suffer the same
pain: search the whole filesystem first, then end of steal blocks every
time.

Anyway maybe by that moment the there are too many open files for
writes(so the fs is full of reservations) so it is already in trouble.

> 
> This work doesn't help us with the slowly-growing logfile or mailbox file
> problem.  I guess that would require on-disk reservations, or a new
> `chattr' hint or such.

Ted has suggested to preserve the reservation/preallocation for those
slowing growing logfile for mailbox file. Probably do not discard the
reservation window for those files(the logfile) when they are closed.
When it opens next time, it will allocate blocks directly from the old
reservation window. Is that what you think?