[CentOS] Wrties starving reads (was: re:Shrinking a volume group)

Steve Bergman steve at rueb.com
Wed Sep 13 18:15:48 UTC 2006

On the topic of writes starving reads in 2.6.9-42.0.2.ELsmp #1 SMP.  It
really *does* lock the box up, for all practical purposes.  It's not
really locked, but any normal user would conclude that the system was
frozen up.

Interestingly enough, I cannot reproduce the writes starving reads
problem on my desktop Ubuntu Dapper box running "2.6.15-26-k7 #1 SMP

This is a UP box without RAID, but with LVM2.  It's an AMD64 4000+,
running 32 bit.

The main differences I see are:

Different kernel version
Ubuntu uses Anticipatory scheduler whereas Centos uses CFQ.

My understanding is that CFQ is supposed to be *better* than
Anticipatory for this kind of thing.

Vanilla is moving to CFQ in 2.6.18 I believe.  Also, the next release of
Ubuntu is moving to CFQ.

I tried moving the CentOS kernel over to my Ubuntu box for testing, but
it is having problems finding the volume groups. 

So, does Linux software Raid1 just suck?  Or CFQ?  Or something else?

CentOS's behavior under heavy writes on this box is bad enough to be
considered a serious bug in my opinion.


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