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SubjectRe: XFS and journalling filesystems
"Vilain, Sam" wrote:
> As a dangerous rule of thumb, LOC ~ code size. More code size = bigger
> kernel = less (buffercache|user memory). <flamesuit>This is a fear of Linux
> kernel developers - Linux ending up as slow as say, Solaris on low end
> machines (even if it kicks butt on 6144-way SMP).</flamesuit>
> Numbers are often good in arguments like this. ie, how big is the ext2fs
> module under Linux/MIPS, compared to the xfs module under Irix? [Comparing
> with Linux/i386 should probably be avoided, because i386 code is (generally)
> more instructions/word, even if you need a few extra million transistors to
> decode it :)].
> In summary, I guess it is important that the filesystem not only performs at
> incredible speeds on high end SMP, but also it's best and average case
> performance on limited memory and IO machines is also good.

I'm afraid your concerns about kernel code size eating into buffer
cache or user memory are slightly out of date. These days it seems
damn near impossible to buy anything with less than 32 MB memory!

When we started VxFS development at Veritas in 1990 under contract
to AT&T Bell Labs, one of the design goals/constraints was that it
should be able to run on machines with 4 MB memory. But by the time
we finished release 1.0 one year or so later, the smallest machine
one could buy already had 8 MB memory. Using another 100kB or so of
kernel memory for a good quality file system is a very reasonable
tradeoff. Or would you rather save 100 kB memory and have a file
system that takes 5, 10 or 20 minutes to recover after a crash?


Dan Koren
Engineering Manager, File Systems phone: (USA) 650-933-3678
Silicon Graphics, Inc. pager: (USA) 888-769-0874
1600 Amphiteatre Pkwy. M/S 08U-500 or
Mountain View, CA 94043-1351 fax: (USA) 650-933-3542

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