Maximum blocksize for FFS?

[Kostik Belousov]

On Mon, Dec 12, 2011 at 06:50:33PM -0500, Dieter BSD wrote:
> Many recent disks have a 4KiB sector size, so newfs's default
> 2KiB frag size seems suboptimal for these drives. Newfs's man
> page states: "The optimal block:fragment ratio is 8:1. Other
> ratios are possible, but are not recommended, and may produce
> poor results."  (It is not clear to me what the 8:1 ratio optimizes,
> or exactly what poor results one should expect with a different ratio?)
> Thus one would logically think of using 32 KiB blocksize 4KiB fragsize
> at a minimum with these drives.
> 
> But, from a discussion in 2009:
> 
> Bruce Evans wrote:
> > Any block size above the default (16K) tends to thrash and fragment buffer
> > cache virtual memory.  This is obviously a good pessimization with lots of
> > small files, and apparently, as you have found, it is a good pessimization
> > with a few large files too.  I think severe fragmentation can easily take
> > several seconds to recover from.  The worst case for causing fragmentaion
> > is probably a mixture of small and large files.
> 
> This caused a *severe* performance problem and I was forced to reduce to
> reduce my blocksize to 16KiB.  :-(
> 
> For data filesystems with large files (multi GB), there are many advantages
> to using large blocksizes such as less space wasted on bookkeeping,
> and faster fsck times.
> 
> So I'm wondering if this buffer cache virtual memory thrashing/fragmenting
> problem has been fixed?  Is it safe to use 64KiB/8KiB yet?  IIRC I've
> read concerns about thrashing/fragmenting due to different filesystems
> having different sizes, say one filesystem being 16K/2K and another being
> 64k/8K?
> 
> Also, has the "bug in vfs_bio.c" been fixed? (64KiB blocksize can
> hang the system)
> 
> Any other gottchas?
I think the default KVA allocated for the buffer cache might be too small
for the 64KB blocks. The only known issue that prevented defragmentation
from working was fixed in r189878.

I did not see further reports of the hangs caused by fragmented buffer KVA.
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