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

[ronnie sahlberg]

List, Kevin,

Since this merge :
commit 1f8bcac09af61e58c5121aa0a932190700ad554d
Merge: cb4c254 1042ec9
Author: Anthony Liguori <address@hidden>
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.


regards
ronnie sahlberg