Re: [Qemu-devel] qcow2x

[Kevin Wolf <kwolf@redhat.com>]

Am 02.08.2011 16:30, schrieb 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

Do you have a public git repo of your code?

> - 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

Yes, I am well aware of that. In my old coroutine-devel branch (it's
still online) I started pushing the locks down and parallelising things
this way. The code that is there is broken because the locking isn't
completely right (L2 allocation vs. cache users of that L2, and somthing
with refcounts). The changes to the cache should be about right, though.

> - L2 allocation can be done with relative data (this is not easy to do
> with current code)

What do you mean by that?

> - 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)

Note that qemu-iotests is often a good indicator, but the tools often
show different behaviour from real guests, so you should also run
benchmarks in a VM.

>     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) ?

You'll need to split it up into reviewable pieces. But let me have a
look at your git tree first.

Are you in the #qemu IRC channel? I think we should coordinate our qcow2
work a bit in order to avoid conflicting or duplicate work.

Kevin