Re: [Qemu-devel] Weird iscsi/fd-event issue since recent merge of event system changes

[Kevin Wolf <kwolf@redhat.com>]

Am 21.05.2012 08:07, schrieb ronnie sahlberg:
> List, Kevin,
> 
> Since this merge :
> commit 1f8bcac09af61e58c5121aa0a932190700ad554d
> Merge: cb4c254 1042ec9
> Author: Anthony Liguori <aliguori@us.ibm.com>
> Date:   Mon Apr 23 14:27:04 2012 -0500
> 
>     Merge remote-tracking branch 'kwolf/for-anthony' into staging
> 
>     * kwolf/for-anthony: (38 commits)
>       qemu-iotests: Fix test 031 for qcow2 v3 support
>       qemu-iotests: Add -o and make v3 the default for qcow2
>       qcow2: Zero write support
>       qemu-iotests: Test backing file COW with zero clusters
>       qemu-iotests: add a simple test for write_zeroes
>       qcow2: Support for feature table header extension
>       qcow2: Support reading zero clusters
>       qcow2: Version 3 images
>       qcow2: Ignore reserved bits in check_refcounts
>       qcow2: Ignore reserved bits in refcount table entries
>       qcow2: Simplify count_cow_clusters
>       qcow2: Refactor qcow2_free_any_clusters
>       qcow2: Ignore reserved bits in L1/L2 entries
>       qcow2: Fail write_compressed when overwriting data
>       qcow2: Ignore reserved bits in count_contiguous_clusters()
>       qcow2: Ignore reserved bits in get_cluster_offset
>       qcow2: Save disk size in snapshot header
>       Specification for qcow2 version 3
>       qcow2: Fix refcount block allocation during qcow2_alloc_cluster_at()
>       iotests: Resolve test failures caused by hostname
>       ...
> 
> I am seeing some weirdness when using iscsi.
> I have isolated it to this particular commit, but since it is 3900
> lines in sinze I have not yet found the exact change that triggers
> this particular behaviour.
> 
> It shows up when using an iscsi device to boot from, where when during
> the bios boot and later grub boot almost all I/O has a pause of 55ms
> between them.
> 
> During boot the bios and later grub will read a lot of data, primarily
> sequentially and one single block at a time.
> After these changes were applied there is now an approximate 55ms
> delay between all these I/O, causing the boot process to become very
> slow.
> 
> 
> I have not yet found the exact part of this big patch that cause this
> slowdown, but will continue investigating.
> 
> I am posting this here in case someone has any idea  or if 55ms rings any bells.

The only thing in this merge that looks as if it could be related is
Paolo's AIO changes between commit adfe92f6 and 9eb0bfca.

Or if you use IDE, Stefan's patches to make PIO operations asynchronous
could be related.

Kevin