linux-ext4 - Re: [PATCH 3/3] add support to check a journal checksum v1 while journal dump.

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Date:   Thu, 22 Nov 2018 08:08:39 +0300
From:   Alexey Lyashkov <alexey.lyashkov@...il.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: [PATCH 3/3] add support to check a journal checksum v1 while
 journal dump.

it’s typical option for Sonexion storages.

journal checksum is enough to cover a corruptions for external journal, 
but provide a less overhead than full metadata checksums.
In our’s workload - any checksums is source of huge latency, so minimal option had enabled.
from other sides it option can enabled without disk format change.

I was have plan to add v2/v3 checksums to patch series but images need to prepared for it.

In this particular usage it was used to find a bug in jbd2 code, with back porting commit
de92c8caf16ca84926fa31b7a5590c0fb9c0d5ca
    jbd2: speedup jbd2_journal_get_[write|undo]_access()

to rhel7 code.

that commits introduce a race window while frozen buffer had modified in parallel thread.
this race likely to be fixed by
ee57aba159a5c329dc78c181a3ae0549e59f0925
    jbd2: simplify code flow in do_get_write_access()

which described as cleanup.

> 22 нояб. 2018 г., в 3:07, Theodore Y. Ts'o <tytso@....edu> написал(а):
> 
> On Mon, Nov 19, 2018 at 12:16:50PM +0300, alexey.lyashkov@...il.com wrote:
>> 
>> journal checksum v1 cover an all blocks in journal descriptor.
>> This checksum stored into commit block and check with commit
>> block reading.
> 
> I'm really curious --- why do you care about journal_checksum v1 at
> all?  It rarely used, and the journal_checksum v3 is much superior.
> 
>      	 	       	     	     		      - Ted
>