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From: yodaiken@freal.cs.umass.edu (victor yodaiken)
Newsgroups: comp.arch
Subject: Re: Mixing paging and IO is inefficient (was Re:  Compiler partions)
Message-ID: <16450@dime.cs.umass.edu>
Date: 5 Jul 90 02:59:56 GMT
References: <499@garth.UUCP> <5660@titcce.cc.titech.ac.jp> <137770@sun.Eng.Sun.COM> <103@array.UUCP>
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Reply-To: yodaiken@freal.cs.umass.edu (victor yodaiken)
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In article <103@array.UUCP> colin@array.UUCP (Colin Plumb) writes:
>In article <137770@sun.Eng.Sun.COM> lm@sun.UUCP (Larry McVoy) writes:
>>I'm really getting sick of this thread.  Those who understand file system
>>semantics dismissed this idea as flawed from the start.  The synchronous
>>nature of certain file system writes are *required* for file system
>>reliability.
...
>>If you must have a software solution now, I'm afraid that you are stuck with
>>the tmpfs method of doing business.
>
>See "Reimplementing the Cedar File System Using Logging and Group
>Commit", Robert Hagmann, Proc. 11th ACM Symp. on Operating Systems
>Principles, also known as ACM Operating Systems Review vol.21 no.5
>(1987).  You don't need extra hardware.
>-- 
>	-Colin

Also see the paper by Borg et al 10th ACM Symp on O.S. on the Auragen
Fault Tolerant Unix, and see the paper on hints for developers by
Butler Lampson in the same proceedings (might be the 9th come to think of
it). If you maintain a valid fs on the disk at all times, you can avoid
synchronous writes and still maintain fs integrity.  Auragen used a simple
technique, the fs root block (super-block) was duplicated. Block A pointed
to a safe version of the FS, block B was kept in memory, and on disk, using
the free blocks of A for writing. When B was safe, i.e. when everything
necessary was on disk, B wold be copied to A. If the system died while
B was not safe, on reboot A would point to a safe fs, with new data written
on its free list (where it would be ignored). Logging was also used to allow
recovery. Detailed information on this fs is in a tech report that I might
be able to find,if anyone is interested.