Power Down

Stephen C. Tweedie sct at redhat.com
Fri Dec 10 05:41:45 EST 1999


On Wed, 08 Dec 1999 09:54:12 +0000, David Woodhouse
<dwmw2 at infradead.org> said:

> Orono at m-sys.com said:
>> The only way to be resistant to power failures in the file system
>> level  is to use a log-structured file system.

>> I heard ext3 is log-structured but I am sure one of the Linux guys here
>> knows more about this, 

> ext3 is a journalling filesystem. I believe that it's not log-structured. 


> Journalling is sufficient for protection from power failures. Stephen, 
> would you care to elaborate on the difference?

Log-structured disks are, oddly enough, structured as a log!  The
entire data layout is in the form of a conceptually infinite log.  All
disk writes are performed at the head of the log, so writes are
necessarily sequential.  The write performance, as a result, is
unmatched.  The read performance can sometimes suffer, but these
filesystems are intended for use with huge amounts of cache, so that
the write performance is more important: most reads are from cache.

A journaled filesystem has a log *in addition to* the normal disk
structure.  The log is used for recovery after a crash but does not
store any long-term persistent data for the filesystem.

> I believe that NTFS is also journalling but not log-structured.



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