Date: Thu, 21 Sep 2000 11:49:23 -0400 (EDT) From: Christopher Stein <stein@eecs.harvard.edu> To: Craig A Soules <soules+@andrew.cmu.edu> Cc: freebsd-fs <freebsd-fs@FreeBSD.ORG> Subject: Re: Journaling Filesystems in bsd? (LFS, anyone?) Message-ID: <Pine.BSF.4.21.0009211119130.11565-100000@orvieto.eecs.harvard.edu> In-Reply-To: <wtmXMoK00UwCIZndEl@andrew.cmu.edu>
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On Thu, 21 Sep 2000, Craig A Soules wrote: > Excerpts from internet.computing.freebsd.fs: 21-Sep-100 Re: Journaling > > Log-structured file systems offer different semantics than > > synchronous journaling file systems. Synchronous journaling can > > offer the traditional durability of create. Nothing is durable > > Wouldn't it be possible to offer the same semantics as FFS in an LFS > implementation if the segment was (over)written after each operation? Partial segment writes? A partial segment write solution as was done in de Jonge & Kaashoek's logical disk. This would solve the internal fragmentation problem and make the cleaner's life easier, while allowing the system to provide traditional UFS create semantics. However, forgetting about the cleaner for a moment, I think performance would be just about the same as the application doing an explicit fsync() to force the full segment. As you said, write times are not dominated by bandwidth so 64KB and 8KB disk writes are probably pretty close. If we write just a portion of the segment the cost will be similar to writing the full segment. So fsyncing full (with lots of internal free space) segments and partial segment writes will be basically the same -- with the important difference being on-disk internal fragmentation. Now bringing the cleaner back into the picture (as it always should be).. the higher level of on-disk fragmentation would drop into run-time performance. The cleaner would be more busy copying and packing segments - generally consuming resources and getting in the way. So I agree that partial segment writes make sense. For the reason that it can offer durability without internal fragmentation - making the cleaner's life easier. -Chris To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-fs" in the body of the message
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