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

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Message-ID: <yq18w1qp3bl.fsf@sermon.lab.mkp.net>
Date:	Fri, 22 Oct 2010 13:03:42 -0400
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	Ric Wheeler <ricwheeler@...il.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.

>>>>> "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

-- 
Martin K. Petersen	Oracle Linux Engineering
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