NAUTOUP BDFLUSHR disk performance tuning

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From: Bela Lubkin <belal@caldera.com>
Subject: Re: Long pauses
Date: Tue, 16 Apr 2002 18:59:01 GMT
References: <ubnr5bqdms3m73@corp.supernews.com> <20020416030233.B6944@mammoth.ca.caldera.com> <ubo57dpd6q3q13@corp.supernews.com> 

[possible FAQ material in this msg and its grandparent?]

Scott wrote:

> Thank you very much for your comprehensive reply!
> I will try bringing BDFLUSH down much lower, and putting NAUTOUP back to 10.
> Hopefully, I'll be able to reboot tonight (if I can manage to kick all the
> users off for long enough!), and I'll let you know what happens tomorrow!

Ok.  FYI, both of these parameters can safely be tuned on a running
system.  (This is not _generally_ true of all parameters, but I've
looked carefully at the kernel source that uses these two parameters and
it both re-acquires the values from the main tunable variables, and uses
them in a safe manner which won't be harmed by dynamic changes.)

To change them, you can use /etc/scodb:

  # scodb -w
  scodb> tune.t_bdflushr=1
  scodb> v.v_autoup=A
  scodb> q
                    ^    ^   or whatever values you want; note: hexadecimal!

You could also use /etc/pat, which is in tls613 at
ftp://stage.caldera.com/TLS/:

  # pat -n /unix tune+20  ........ = 00000001 # tune.t_bdflushr=1: run bdflush often
  # pat -n /unix v+4c     ........ = 0000000A # v.v_autoup=10: flush dirty bufs after 10s

`pat` has the advantage that you can put comments into the flow of
control.  It has the fairly large disadvantage that it doesn't know the
shapes of the structures, so you have to manually compute them and could
mistakenly patch the wrong field.

If you patch BDFLUSHR, bdflush will wake up after the last sleep at the
old BDFLUSHR, then start sleeping at the new rate.  So for instance you
may not notice the once-a-second write cycle until up to old-BDFLUSHR
seconds have passed.  I believe you could also force the issue by doing
a `sync` immediately after tuning it.

If you patch NAUTOUP, buffers already in the buffer cache _will_ be
affected -- it is checked by bdflush() whenever it wakes up -- the
timeout is a property considered by the bdflush algorithm rather than a
property stored in each individual buffer.  (I suppose you could
dynamically tune these parameters to deal with external conditions --
set both to 1 during a lightning storm, trading performance for minimal
data loss in the event of a catastrophic failure?)

>Bela<
 

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