Re: Tracing block devices (was: Re: [Qemu-devel] Static tracepoint control via trace-event)

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

On Tue, Oct 19, 2010 at 03:29:51PM +0100, Richard W.M. Jones wrote:
> On Tue, Oct 19, 2010 at 03:59:36PM +0200, Jan Kiszka wrote:
> > Once we have "-trace events=...", defining the list of active
> > tracepoints before starting qemu will be trivial (e.g. via a config
> > file). Of course, this requires that all tracepoints are built-in...
> 
> Sorry that I've not been following this very closely, but does this
> sort of thing allow tracing reads and writes to block devices?  Am I
> right in thinking that if a tracepoint existed in the right place, one
> could get a log file from that which could be post-processed in
> another tool?
> 
> cf:
> http://rwmj.wordpress.com/2010/10/05/visualizing-reads-writes-and-alignment/#content

Definitely, here is the commit that added bdrv_aio_writev/bdrv_aio_readv
tracing.  bdrv_aio_multiwrite has been traced for a while.

http://patchwork.ozlabs.org/patch/66843/

As an example, I use the following script to find all write requests
that touch a given region.  This is very useful for debugging image
corruptions given a trace file:

The usage is:

find_overlapping_io.py <bs> <sector_num> <nb_sectors>

where bs is the block driver state pointer, sector_num is the starting
sector address, and nb_sectors is the number of sectors.

#!/usr/bin/env python
import sys

def trace_filter(fobj, event, keys):
    for line in fobj:
        fields = line.strip().split()
        if fields[0] != event:
            continue

        attrs = dict([(k, v) for k, v in (x.split('=') for x in fields[2:])])
        match = True
        for k, v in keys.iteritems():
            if k not in attrs:
                match = False
                break
            if attrs[k] != v:
                match = False
                break

        if match:
            yield attrs

def intersection(a_sector_num, a_nb_sectors, b_sector_num, b_nb_sectors):
    return not (a_sector_num + a_nb_sectors <= b_sector_num or \
                b_sector_num + b_nb_sectors <= a_sector_num)

bs, sector_num, nb_sectors = sys.argv[1:]
sector_num = int(sector_num, 0)
nb_sectors = int(nb_sectors, 0)

for req in trace_filter(sys.stdin, 'bdrv_aio_writev', {'bs': bs}):
    if intersection(sector_num, nb_sectors, int(req['sector_num'], 0), int(req['nb_sectors'], 0)):
        print req

Stefan