linux-kernel - Re: [PATCH 00/11] Add compression support to pstore

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Date:	Wed, 7 Aug 2013 15:22:45 -0700
From:	Tony Luck <tony.luck@...il.com>
To:	Aruna Balakrishnaiah <aruna@...ux.vnet.ibm.com>
Cc:	"linuxppc-dev@...abs.org" <linuxppc-dev@...abs.org>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"keescook@...omium.org" <keescook@...omium.org>
Subject: Re: [PATCH 00/11] Add compression support to pstore

On Tue, Aug 6, 2013 at 10:35 PM, Tony Luck <tony.luck@...il.com> wrote:
> ERST is at the whim of the BIOS writer (the ACPI standard doesn't provide any
> suggestions on record sizes).  My systems support ~6K record size.

Off by a little - 7896 bytes on my current machine.

> efivars has, IIRC, a 1k limit coded in the Linux back end.
My memory was correct for this one.

Adding a little tracing to pstore_getrecords() I see this:

pstore: inflated 3880 bytes compressed to 17459 bytes
pstore: inflated 2567 bytes compressed to 17531 bytes
pstore: inflated 4018 bytes compressed to 17488 bytes

Which isn't at all what I expected.  The ERST backend
advertised a bufsize of 7896, and I have the default
kmsg_bytes of 10240.  So on my forced panic the code
decided to create a three part pstore dump.  The sum of
the pieces is close to, but a little over the target of 10K.
But I don't understand why the compressed sizes are so
much smaller that the ERST backend block size.

The uncompressed sizes appear to be close to constant.
The compression ratios vary from 14% to 23%

Why do we get three small parts instead of two bigger
ones close the the 7896 ERST bufsize?

-Tony
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