Date: Fri, 24 Oct 2008 18:18:36 +0200 From: Thierry Herbelot <thierry@herbelot.com> To: hackers@freebsd.org Cc: freebsd-fs@freebsd.org Subject: question about sb->st_blksize in src/sys/kern/vfs_vnops.c Message-ID: <200810241818.37262.thierry@herbelot.com>
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Hello,
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
http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/kern/vfs_vnops.c.diff?r1=1.210;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 ?
TfH
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