Subject: LFS "smooth" syncer workaround/performance tuning
To: None <,>
From: Blair Sadewitz <>
List: tech-perform
Date: 04/01/2007 21:15:54
I'd grown annoyed at how LFS would blast data to the disk according to
vfs.sync.*delay settings. A while ago, I thought, "why not just
disable the sync delay altogether?"  Try setting:

vfs.sync.delay = 0
vfs.sync.filedelay = 0
vfs.sync.metadelay = 0
vfs.sync dirdelay = 0

Then, find out how much bandwidth you have at the inside and outside
of your disk.  That is, run newfs_lfs -ANF on a partition close to
cylinder 0, then do the same on a partition close to the last
cylinder.  Average the two, then calculate the LFS "page trip" with
the following formula (thanks tls):

t=(avg_bandwidth_in_bytes / PAGE_SIZE) / 4

I'd imagine that averaging is more or less moot if you don't plan on
using > %50 of your disk's capacity, or if you are using a partition
smaller than a majority of the disk.

Set vfs.lfs.pagetrip=t, and notice how data is now written much more
smoothly to the disk.  You can fiddle with vfs.lfs.pagetrip according
to the amount of latency you want.  This works especially well with
BUFQ_PRIOCSCAN or BUFQ_READPRIO.  For further optimization, I'd
recommend using BUFQ_PRIOCSCAN and tweaking the burst values using
xtraeme's bufq sysctl patch:

I'd be interested to hear if other people find this useful or not.  I
find these settings useful enough such that I wish there were a way to
turn off vfs sync only for LFS, as using these settings with FFS isn't
too much fun. ;)



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