[Qemu-devel] qcow2x

[Frediano Ziglio]

Hi,
  I spent some time trying to find a way to speed up qcow2 performance
on allocation and snapshot so I branch from kevin/coroutine-block
branch a qcow2x branch. Currently it just write using different
algorithm (that is is fully compatible with qcow2, there is not
ICountAsZero(TM) method :) ). Mainly is a complete rewrite of
qcow2_co_writev. Some problems I encountered in the way current qcow2
code works

- reference decrement are not optimized (well, this is easy to fix on
current code)
- any L2 update blocks all other L2 updates, this is a problem if
guest is writing large file sequentially cause most write needs to be
serialized
- L2 allocation can be done with relative data (this is not easy to do
with current code)
- data/l2 are allocated sequentially (if there are not hole freed) but
written in another order. This cause excessive file fragmentation with
default cache mode, for instance on xfs file is allocated quite
sequentially on every write so any no-sequential write create a
different fragment.

Currently I'm getting these times with iotests (my_cleanup branch is
another branch more conservative with a patch to collapse reference
decrement, note that 011 and 026 are missing, still not working)

    X    C    B
001 6    3    7
002 3    3    4
003 3    3    3
004 0    1    0
005 0    0    0
007 35   32   36
008 3    4    3
009 1    0    0
010 0    0    0
012 0    0    2
013 125  err  158
014 189  err  203
015 48   70   610
017 4    4    4
018 5    5    5
019 4    4    4
020 4    4    4
021 0    0    0
022 74   103  103
023 75   err  95
024 3    3    3
025 3    3    6
027 1    1    0
028 1    1    1

X qcow2x
C my_cleanup
B kevin/coroutine-block

Currently code is quite "spaghetti" code (needs a lot of cleanup,
checks, better error handling and so on). Taking into account that
code require additional optimizations and is full of internal
debugging time times are quite good.

Main questions are:
- are somebody interesting ?
- how can I send such a patch for review considering that is quite big
(I know, I have to clean a bit too) ?

Regards,
  Frediano