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

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Date:	Fri, 22 Oct 2010 13:14:12 -0400
From:	Ric Wheeler <ricwheeler@...il.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
CC:	Eric Sandeen <sandeen@...hat.com>,
	Lukas Czerner <lczerner@...hat.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH] e2fsck: Discard free data and inode blocks.

  On 10/22/2010 01:03 PM, Martin K. Petersen wrote:
>>>>>> "Ric" == Ric Wheeler<ricwheeler@...il.com>  writes:
>>> so we're not depending on it zeroing blocks, we're just depending on
>>> it advertising correctly whether or not it -does- zero.
> If the relevant bits are set (ATA: DRAT and RZAT, SCSI: TPRZ) we'll set
> the bdev's discard_zeroes_data flag.
>
> [root@...t ~]# lsscsi | grep SSD | awk '{ print $7 }'
> /dev/sde
> [root@...t ~]# grep . /sys/block/sde/queue/discard_zeroes_data
> 1
>
> The relevant ioctl is BLKDISCARDZEROES.
>
>
> Ric>  I think that ATA devices have historically not done this correctly,
>
> I'm only aware of one drive that advertised RZAT and got it wrong. I
> believe a firmware update fixed it.
>
> Generally we assume that if the firmware writers go through the effort
> of reporting things correctly then they have also implemented the
> feature. We have quite a few sanity checks in place in libata so we
> won't trigger if the firmware guys just put all ones in a word, for
> instance. There are several things that need to line up for us to
> actually set the discard flags.
>
>
> PS. http://oss.oracle.com/~mkp/docs/linux-advanced-storage.pdf
>

Hi Martin,

I think that the T13 people are still reworking (or have just finished?) the 
spec. For example,

http://www.t13.org/documents/UploadedDocuments/docs2010/e09158r2-Trim_Clarifications.pdf

has more detail on how to interpret those bits.  I certainly cannot claim to 
have followed all of this recently, but do recall that early on the ATA devices 
specifically had various behaviors that caused concern.

Ric

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