linux-kernel - Re: [PATCH 10/21] block: Add fops atomic write support

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Message-ID: <d976868a-d32c-43d1-b5da-ebbc4c8de468@acm.org>
Date:   Thu, 5 Oct 2023 15:58:38 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Dave Chinner <david@...morbit.com>
Cc:     "Martin K. Petersen" <martin.petersen@...cle.com>,
        John Garry <john.g.garry@...cle.com>, axboe@...nel.dk,
        kbusch@...nel.org, hch@....de, sagi@...mberg.me,
        jejb@...ux.ibm.com, djwong@...nel.org, viro@...iv.linux.org.uk,
        brauner@...nel.org, chandan.babu@...cle.com, dchinner@...hat.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
        linux-api@...r.kernel.org
Subject: Re: [PATCH 10/21] block: Add fops atomic write support

On 10/5/23 15:36, Dave Chinner wrote:
> $ lspci |grep -i nvme
> 03:00.0 Non-Volatile memory controller: Samsung Electronics Co Ltd NVMe SSD Controller SM981/PM981/PM983
> 06:00.0 Non-Volatile memory controller: Samsung Electronics Co Ltd NVMe SSD Controller SM981/PM981/PM983
> $ cat /sys/block/nvme*n1/queue/write_cache
> write back
> write back
> $
> 
> That they have volatile writeback caches....

It seems like what I wrote has been misunderstood completely. With
"handling a power failure cleanly" I meant that power cycling a block device
does not result in read errors nor in reading data that has never been written.
Although it is hard to find information about this topic, here is what I found
online:
* About certain SSDs with power loss protection:
   https://us.transcend-info.com/embedded/technology/power-loss-protection-plp
* About another class of SSDs with power loss protection:
   https://www.kingston.com/en/blog/servers-and-data-centers/ssd-power-loss-protection
* About yet another class of SSDs with power loss protection:
   https://phisonblog.com/avoiding-ssd-data-loss-with-phisons-power-loss-protection-2/

So far I haven't found any information about hard disks and power failure
handling. What I found is that most current hard disks protect data with ECC.
The ECC mechanism should provide good protection against reading data that
has never been written. If a power failure occurs while a hard disk is writing
a physical block, can this result in a read error after power is restored? If
so, is this behavior allowed by storage standards?

Bart.