linux-ext4 - Re: [PATCH] e2fsck: Discard free data and inode blocks.

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Message-Id: <6C34898A-508C-4140-A494-B279C04EDD50@dilger.ca>
Date:	Fri, 22 Oct 2010 12:17:31 -0600
From:	Andreas Dilger <adilger.kernel@...ger.ca>
To:	Lukas Czerner <lczerner@...hat.com>
Cc:	Ric Wheeler <ricwheeler@...il.com>, linux-ext4@...r.kernel.org,
	tytso@....edu, sandeen@...hat.com
Subject: Re: [PATCH] e2fsck: Discard free data and inode blocks.

On 2010-10-22, at 12:01, Lukas Czerner wrote:
>> That patch also checks for the zeroing feature.  When this patch was first under discussion, I proposed that we validate that the device is actually zeroed by doing a write a non-zero block to the disk and then calling discard+zero for that region, and reading back the block and verifying it.
>> 
>> Eric wasn't convinced that was necessary, maybe you can convince him more...
> 
> One of the counter arguments was, that some devices does not preserve
> this behavior through power cycles. I think Ted was the one talking
> about that.

Sure, I don't think we can handle every pathology, but doing a write/discard/read of a few blocks (when it has the potential to avoid many GB of writes for zeroing) is surely easy and worthwhile?

In any case, I thought that discussion was about a device that didn't report BLKDISCARDSZEROES=1, but only that a normal DISCARD would read back zero until the next restart?  That prevents optimizations like "read until we see non-zero data, then start writing zeroes", which would still be faster for many RAID devices (or older kernels that don't have DISCARD/ZERO support at all).

Cheers, Andreas

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