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Re: [zfs-discuss] Periodic flush

Subject: Re: [zfs-discuss] Periodic flush
From: Bob Friesenhahn
Date: Thu, 27 Mar 2008 10:08:56 -0500 CDT
On Wed, 26 Mar 2008, Neelakanth Nadgir wrote:
> When you experience the pause at the application level,
> do you see an increase in writes to disk? This might the
> regular syncing of the transaction group to disk.

If I use 'zpool iostat' with a one second interval what I see is two 
or three samples with no write I/O at all followed by a huge write of 
100 to 312MB/second.  Writes claimed to be a lower rate are split 
across two sample intervale.

It seems that writes are being cached and then issued all at once. 
This behavior assumes that the file may be written multiple times so a 
delayed write is more efficient.

If I run a script like

while true
do
sync
done

then the write data rate is much more consistent (at about 
66MB/second) and the program does not stall.  Of course this is not 
very efficient.

Are the 'zpool iostat' statistics accurate?

Bob
======================================
Bob Friesenhahn
bfriesen@xxxxxxxxxxxxxxxxxxx, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

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