From mboxrd@z Thu Jan  1 00:00:00 1970
From: NeilBrown <neilb@suse.de>
Subject: Re: RAID5 hard freeze
Date: Tue, 25 Feb 2014 13:58:09 +1100
Message-ID: <20140225135809.0b1afc69@notabene.brown>
References: <CAF8Fvyz2PnEd=t=E-xUuG6anoLhm1zJCn0txD=P_aE_5AmoVUw@mail.gmail.com>
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To: Denis Golovan <denis.golovan@gmail.com>
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On Tue, 25 Feb 2014 00:01:42 +0200 Denis Golovan <denis.golovan@gmail.com>
wrote:

> Hi all
>=20
> I am struggling to diagnose a strange freeze of software RAID5 array.
> My RAID5 consists of 4 Toshiba SATA drives and has ext4 filesystem on top=
 of it.
>=20
> It works fine unless I start several process writing intensively to it.
> At first, it looks like the system is under high pressure, then the
> system starts lagging a lot and a hard freeze always follows after
> several minutes.
>=20
> No errors in system log, nothing is emitted to console. Just hard
> freeze with HDD light always on. I tried enabling kernel network
> logging to another machine and again no information when hanging.
> After reboot, my array starts reconstruction and finishes without
> errors.
>=20
> I tried disabling quotas and barriers for ext4.
> After disabling barriers, it almost seemed to work, but after some
> time the same hard freeze happens.
>=20
> I tested the same hardware configuration under Linux v3.10, 3.11, 3.12
> and now 3.13.5 (all x86 arch) behaves the same way. The same issue can
> be reproduced easily.
>=20
> So now I tested everything Google suggests on the matter.
> Could you give a hint on how to debug this issue?
>=20

The most useful thing for debugging a hard freeze is the alt-sysrq-T output
when it is frozen.  typing that magic sequence should always produce some
output unless it is hard-frozen with interrupts disabled.

So make sure you can produce the output when the system is working properly
(to a log file file the network console would be ideal), then when it hangs,
produce the output again.
To probably need to have a text console rather than a graphic console for it
to work.


If it is hard-hanging with interrupts disabled, then it gets tricky.  I
thought there was some NMI-based lockup detector which would warn if that
happened, but I cannot find it just now.

NeilBrown

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