Tejun Heo wrote:
Ric Wheeler wrote:
May 17 03:30:54 centera kernel: [start_ordered ]: BIO
d08998c0 3357330 4096
May 17 03:30:54 centera kernel: [start_ordered ]: ordered=1
in_flight=0
May 17 03:30:54 centera kernel: [blk_do_ordered ]:
start_ordered ed3553c0->dfe2bd28
May 17 03:30:54 centera kernel: [elv_next_request ]: dfe2bd28
(bar)
May 17 03:30:54 centera kernel: [ordered_bio_endio ]:
q->orderr=0 error=0
May 17 03:30:54 centera kernel: [flush_dry_bio_endio ]: BIO
f18ffa00 3357330 4096
May 17 03:30:54 centera kernel: [blk_ordered_complete_seq]: ordseq=17
seq=08 orderr=0 error=0
May 17 03:30:54 centera kernel: [blk_ordered_complete_seq]: sequence
complete
May 17 03:30:54 centera kernel: [ordered_bio_endio ]:
q->orderr=0 error=0
May 17 03:30:54 centera kernel: [flush_dry_bio_endio ]: BIO
f18ff300 3357330 4096
May 17 03:30:54 centera kernel: [blk_ordered_complete_seq]: ordseq=17
seq=08 orderr=0 error=0
May 17 03:30:54 centera kernel: [blk_ordered_complete_seq]: sequence
complete
[--snip--]
Combined with your boot log, everything seems as expected. Write
cache is disabled, so the chosen ordered mode is 0x1 - DRAIN only. In
the log you've posted, there are no pending IOs on barrier issue, so
the barrier is executed directly and then the sequence is complete.
If there were other requests pending, all that happens would be
waiting for them to finish before issuing the barrier.
If you turned on write cache after the machine booted, what method did
you use to turn on write cache? If you issued the command directly
using sg to an active device (usage count >= 1), SCSI doesn't snoop
such commands and thus it won't adjust ordered mode automatically.
However, sd forces disk revalidation (and thus ordered
reconfiguration) if you change cache mode via
/sys/class/scsi_disk/X\:0\:0\:0/cache_type.
That is what I suspected...
The write cache gets set via hdparm (in the previous kernel we had just
hacked the sd code to always assume write back/barriers until this /sys
control came along ;-)).
What is the magic sequence to get the cache type to flip? My first tries
to poke cache type leave it firmly in write through...
ric
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