[ENBD] ENBD on 2.4.x

[Jon Arney]

Hi.

I've spent some time with ENBD and the 2.4 kernel and have a couple
of observations (for what it's worth).

Under 2.4.0-prerelease, I am able to create a network block device
and connect it to client and server and perform many millions of
read/write
requests to it by opening '/dev/nda'.  No problems there.  I can also
run
'mke2fs /dev/nda' and it seems to create a good filesystem.  When I
mount
that filesystem and run 'bonnie' under it, it blocks indefinitely during
the 'Writing Intelligently' phase.  '/proc/nbdinfo' shows me the
following:

Device a:       Open 
[a] State:      verify, rw, enabled, last error 0
[a] Queued:     +0R/0W curr reqs =0R/0W real reqs +1R/128W max reqs
[a] Buffersize: 86016   (sectors=168)
[a] Blocksize:  1024    (log=10)
[a] Size:       32768KB
[a] Blocks:     32768
[a] Sockets:    1       (*)
[a] Requested:  218000  (218000)  
[a] Despatched: 217488  (217488)  
[a] Errored:    0       (0)     0+0
[a] Pending:    0       (0)     0R/0W+0R/512W
[a] Kthreads:   0       (0 waiting/0 running/1 max)
[a] Cthreads:   1       (+)
[a] Cpids:      1       (30584)
Device b-p:     Closed

Note that under 'pending', there are 512 requested writes which have
not been dispatched.  I have not yet traced the cause of this.

Also, under 2.4.1 and above (recently available), the read/write
requests directly
to the device '/dev/nda' block indefinitely and 'mke2fs' blocks
indefinitely
(not allowing me to continue with the mount and bonnie test).  I believe
that I
have discovered the cause of this problem.  It seems that in 2.4.1, some
changes to the
'elevator' optimizations were made and this code seems to be in a state
of flux
for the moment.
May I suggest that for the moment, we consider adding a line here at
approximately
line 3490 of 'nbd.c' in the Kernel driver:

#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,3,30)
// JSA - Add this line because under 2.4.1 and above, the merge
optimizations are in flux
#if LINUX_VERSION_CODE < KERNEL_VERSION(2.4.1)
    // PTB control merge attempts so we don't overflow our buffers
    ll_merge_requests_fn =
(BLK_DEFAULT_QUEUE(MAJOR_NR))->merge_requests_fn;
    ll_front_merge_fn    =
(BLK_DEFAULT_QUEUE(MAJOR_NR))->front_merge_fn;
    ll_back_merge_fn     = (BLK_DEFAULT_QUEUE(MAJOR_NR))->back_merge_fn;
    (BLK_DEFAULT_QUEUE(MAJOR_NR))->merge_requests_fn =
&nbd_merge_requests_fn;
    (BLK_DEFAULT_QUEUE(MAJOR_NR))->front_merge_fn    =
&nbd_front_merge_fn;
    (BLK_DEFAULT_QUEUE(MAJOR_NR))->back_merge_fn     =
&nbd_back_merge_fn;
#endif
#endif

Finally, (not to be picky), but 'Dispatch' is spelled with an 'i', not
an 'e' :)

Thank you all for your time and consideration.

Sincerely,
Jonathan S. Arney

------------------------------------------------------------------------
Ensor Computing                          http://www.ensor.org
Home of the Orac supercomputer
------------------------------------------------------------------------