Re: Linux 2.6.29

[Martin Steigerwald <Martin@lichtvoll.de>]

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Am Mittwoch 25 März 2009 schrieb Jesse Barnes:
> On Tue, 24 Mar 2009 09:20:32 -0400
>
> Theodore Tso <tytso@mit.edu> wrote:
> > They don't solve the problem where there is a *huge* amount of writes
> > going on, though --- if something is dirtying pages at a rate far
> > greater than the local disk can write it out, say, either "dd
> > if=/dev/zero of=/mnt/make-lots-of-writes" or a massive distcc cluster
> > driving a huge amount of data towards a single system or a wget over
> > a local 100 megabit ethernet from a massive NFS server where
> > everything is in cache, then you can have a major delay with the
> > fsync().
>
> You make it sound like this is hard to do...  I was running into this
> problem *every day* until I moved to XFS recently.  I'm running a
> fairly beefy desktop (VMware running a crappy Windows install w/AV junk
> on it, builds, icecream and large mailboxes) and have a lot of RAM, but
> it became unusable for minutes at a time, which was just totally
> unacceptable, thus the switch.  Things have been better since, but are
> still a little choppy.
>
> I remember early in the 2.6.x days there was a lot of focus on making
> interactive performance good, and for a long time it was.  But this I/O
> problem has been around for a *long* time now... What happened?  Do not
> many people run into this daily?  Do all the filesystem hackers run
> with special mount options to mitigate the problem?

Well I always had the feeling that somewhen from one 2.6.x to another I/O 
latencies increased a lot. But first I thought I was just imaging this 
and when I more and more thought that this is for real, I forgot since 
when I observed these increased latencies.

This is on IBM ThinkPad T42 and T23 with XFS.

I/O latencies are pathetic when dpkg reads in the database or I do tar -xf 
linux-x.y.z.tar.bz2.

I never got down to what is causing these higher latencies though also I 
tried different I/O schedulers, tuned XFS options, used relatime.

What I found tough is that on XFS at least a tar -xf linux-kernel / rm -rf 
linux-kernel operation is way slower with barriers and write cache 
enabled that with no barriers and no write cache enabled. And frankly I 
never got that.

XFS crawls to a stop on metadata operations when barriers are enabled. 
According to the XFS FAQ disabling drive write cache should be as safe as 
enabling barriers. And I always unterstood barriers as a feature to be 
have *some* ordering contraints, i.e. write before barrier go before 
barrier and writes after it after it - even when a drives hardware write 
cache is involved. But when this cache is enabled ordering will always be 
like issued from Linux block layer cause all I/Os issued to the drive are 
write-through and synchron without write cache, versus only barrier 
requests are synchron with barriers and write cache.

-- 
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
GPG: 03B0 0D6C 0040 0710 4AFA  B82F 991B EAAC A599 84C7

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