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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