Subject: Re: LFS "smooth" syncer workaround/performance tuning
To: Blair Sadewitz <>
From: Juan RP <>
List: tech-perform
Date: 04/03/2007 10:00:20
This patch for bufq+sysctl is not optimal because it should be using values 
not globally, perhaps in the future I'll do it correctly...

----- Original Message ----- 
From: "Blair Sadewitz" <>
To: <>; <>
Sent: Monday, April 02, 2007 3:15 AM
Subject: LFS "smooth" syncer workaround/performance tuning

> 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. ;)