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

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Message-ID: <4CC1D694.3040006@gmail.com>
Date:	Fri, 22 Oct 2010 14:23:16 -0400
From:	Ric Wheeler <ricwheeler@...il.com>
To:	Andreas Dilger <adilger.kernel@...ger.ca>
CC:	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org,
	tytso@....edu, sandeen@...hat.com
Subject: Re: [PATCH] e2fsck: Discard free data and inode blocks.

  On 10/22/2010 02:17 PM, Andreas Dilger wrote:
> 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

Just to further confuse things, if we just want to zero a device, there is the 
(relatively old) WRITE_SAME command that arrays use. Note that it is quite a bit 
faster than doing this from the server since you only transfer over one block of 
data and the disk firmware does the rest - no data transfer for each block once 
you start.

It can certainly take a long, long time, but would be faster than zeroing a 
drive with write() system calls :)

ric

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