question about sb->st_blksize in src/sys/kern/vfs_vnops.c

Thierry Herbelot thierry at
Fri Oct 24 16:36:59 UTC 2008


the [SUBJ] file contains the following extract (around line 705) :

     * Default to PAGE_SIZE after much discussion.
     * XXX: min(PAGE_SIZE, vp->v_bufobj.bo_bsize) may be more correct.

    sb->st_blksize = PAGE_SIZE;

which arrived around four years ago, with revision 1.211 (see;r2=1.211;f=h)

the net effect of this change is to decrease the block buffer size used in 
libc/stdio from 16 kbytes (derived from the underlying ufs partition) to 
PAGE_SIZE ==4 kbytes (fixed value), and consequently the I/O bandwidth is 
lowered (this is on a slow Flash).

I have patched the kernel with a larger, fixed value (simply 4*PAGE_SIZE, to 
revert to the block size previoulsly used), and the kernel and world seem to 
be running fine.

Seeing the XXX coment above, I'm a bit worried about keeping this new 
st_blksize value.

are there any drawbacks with running with this bigger buffer size value ?


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