immense delayed write to file system (ZFS and UFS2), performance issues

[Michael Loftis]

--On Wednesday, January 20, 2010 1:16 AM +0100 "O. Hartmann" 
<ohartman at mail.zedat.fu-berlin.de> wrote:

> This could end in a bad situation, where one process writes a files, say
> with some arbitrary stuff and another successing process is intended to
> read this file. even if the processes are run serial, those 'delays'
> could break the chain! The delay situation in a development environment
> is harsh, but in other circumstances it could develop very bad.

The read occurs from the write cache.  Thus the reader would see the 
writers data (given the usual POSIX semantics).  ZFS delayed writes are 
handled by the cache/ZIL layers, reads and writes go through these layers. 
The ZIL is normally written very quickly.

ZFS actually (through the ZIL) has better journalling and recovery 
semantics than most journalling filesystems.

>
> I see this strange behaviour now for several weeks, something essential
> has changed in the code, I guess.
> On UP boxes the situation is worse sometimes, on SMp boxes with lots of
> RAM ( 8 and 16 GB and 4 or 8 CPU cores) it is still bad. I have a server
> that acts as a 'rsync' backup system gathering data from satellite
> servers from time to time. Since this problem of slowness occured, this
> 4-core 8 gig RAM box crawls for minutes. Even when X11 is disabled
> working on console is 'bumpy': terminal out slows down, mouse pointer
> jumps etc.As I wrote, the same on a 8 core/16 gig box, but not that harsh.
>
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