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Date:      Thu, 7 Oct 1999 10:31:46 +1000 (EST)
From:      Bruce Evans <bde@zeta.org.au>
To:        "Matthew D. Fuller" <fullermd@futuresouth.com>
Cc:        Peter Jeremy <peter.jeremy@alcatel.com.au>, Alfred Perlstein <bright@wintelcom.net>, freebsd-current@FreeBSD.ORG
Subject:   Re: make install trick
Message-ID:  <Pine.BSF.4.10.9910070953020.9776-100000@alphplex.bde.org>
In-Reply-To: <19991006154419.O20768@futuresouth.com>

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> /dev/da0s1a on / (local, synchronous, writes: sync 32 async 15100)
>                          ^^^^^^^^^^^
> 
> Though I'm still waiting for an explanation of WHY exactly I have async
> writes on a sync partition.   Nobody yet has said anything but 'that's
> interesting...'.  A direction to look would be helpful.

The sync flag only affects data writes and block allocation for data
writes.  Metadata is normally always written synchronously, except for
safe optimisations.  Safe optimisations include never writing inode or
block bitmaps (fsck can recover these), not writing some indirect blocks
(correct block numbers are not very useful until the blocks have been
written), and not writing inodes in many cases where only timestamps
have changed.

You seem to have a lot of async writes.  This may be caused by using
soft updates.  Soft updates causes many (or most or all) metadata
updates to be done using delayed writes.  Using the sync flag with
soft updates doesn't make much sense and may be harmfull (there doesn't
seem to be anything to prevent data writes being done long before the
metadata points to the data).

Smaller number of async writes may be caused by metadata write
optimisations.

Bruce



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