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Date: Fri, 12 Jun 1998 11:42:19 +1000
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199806120142.LAA22840@godzilla.zeta.org.au>
To: dyson@FreeBSD.ORG, julian@whistle.com
Subject: Re: cvs commit: src/sys/ufs/ffs ffs_vnops.c
Cc: bde@zeta.org.au, cvs-committers@FreeBSD.ORG, julian@FreeBSD.ORG
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>Kirk wanted the removal of the vfs_bio_awrite for testing various ideas
>It wil likely come back after a while. 
>As John said. it's not much of a loss for indirect blocks.

>> > It also seems to change the semantic to "wait for v_numoutput even in
>> > the soft updates case", and fix some style bugs, and break the
>> > optimization of using vfs_bio_awrite() instead of bawrite() for async
>> > writes of indirect blocks.
>> > 
>> If vfs_bio_awrite optimization is broken for *just* indirect blocks,
>> there isn't much lossage.  If the vfs_bio_awrite is broken for data
>> blocks, the performance loss is significant.

I read the test backwards.  The breakage is actually for MNT_WAIT writes
of non-indirect blocks - i.e., the usual case for MNT_WAIT is pessimized.
We use async writes even in the MNT_WAIT case to start the i/o, and wait
for the i/o to complete before returning from ffs_fsync().  We used to
use vfs_bio_awrite() to optimize these async writes if possible.  Lite2
doesn't have either optimization.

I can't believe you actually want to remove vfs_bio_awrite() for testing.
vfs_bio_awrite() is only slightly less deterministic that bawrite().
For testing and for getting closer to Lite2, you want to use bwrite().

Bruce

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